Encyclopedia Britannica, 9th ed. (1878)
 The Biological sciences are those which deal with the phenomena manifested by living matter; and though it is customary and convenient to group apart such of these phenomena as are termed mental, and such of them as are exhibited by men in society, under the heads of Psychology and Sociology, yet it must be allowed that no natural boundary separates the subject matter of the latter sciences from that of Biology. Psychology is inseparably linked with Physiology; and the phases of social life exhibited by animals other than man, which sometimes curiously foreshadow human policy, fall strictly within the province of the biologist.
On the other hand, the biological sciences are sharply marked off from the abiological, or those which treat of the phenomena manifested by non-living matter, in so far as the properties of living matter distinguish it absolutely from all other kinds of things, and as the present state of knowledge furnishes us with no link between the living and the not-living.
These distinctive properties of living matter are
1. Its chemical compositioncontaining, as it invariably does, one or more forms of a complex compound of carbon, hydrogen, oxygen, and nitrogen, the so-called protein (which has never yet been obtained except as a product of living bodies) united with a large proportion of water, and forming the chief constituent of a substance which, in its primary unmodified state, is known as protoplasm.
2. Its universal disintegration and waste by oxidation; and its concomitant reintegration by the intus-susception of new matter.
A process of waste resulting from the decomposition of the molecules of the protoplasm, in virtue of which they break up into more highly oxidated products, which cease to form any part of the living body, is a constant concomitant of life. There is reason to believe that carbonic acid is always one of these waste products, while the others contain the remains of the carbon, the nitrogen, the hydrogen, and the other elements which may enter into the composition of the protoplasm. The new matter taken in to make good this constant loss is either a ready-formed protoplasmic material, supplied by some other living being, or it consists of the elements of protoplasm, united together in simpler combinations, which consequently have to be built up into protoplasm by the agency of the living matter itself. In either case, the addition of molecules to those which already existed takes place, not at the surface of the living mass, but by interposition between the existing molecules of the latter. If the processes of disintegration and of reconstruction which characterize life balance one another, the size of the mass of living matter remains stationary, while, if the reconstructive process is the more rapid, the living body grows. But the increase of size which constitutes growth is the result of a process of molecular intus-susception, and therefore differs altogether from the process of growth by accretion, which may be observed in crystals and is effected purely by the external addition of new matter so that, in the well-known aphorism of Linnæus, the word "grow," as applied to stones, signifies a totally different process from what is called "growth" in plants and animals
3. Its tenderly to undergo cyclical change.
In the ordinal course of nature, all living matter proceeds from pre-existing living matter, a portion of the latter being detached and acquiring an independent existence. The new form takes on the characters of that from which it arose; exhibits the same power of propagating itself by means of an offshoot; and, sooner or later, like its predecessor, ceases to live, and is resolved into more highly oxidated compounds of its elements.
Thus an individual living body is not only constantly changing its substance, but its size and form are undergoing continual modifications, the end of which is the death and decay of that individual; the continuation of the kind being secured by the detachment of portions which tend to run through the same cycle of forms as the parent. No forms of matter which are either not living, or have not been derived from living matter, exhibit these three properties, nor any approach to the remarkable phenomena defined under the second and third heads. But in addition to these distinctive characters, living matter has some  other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon heat, within a limited range of temperature, and the fact that it usually possesses a certain structure, or organization.
As has been said, a large proportion of water enters into the composition of all living matter; a certain amount of drying arrests vital activity, and the complete abstraction of this water is absolutely incompatible with either actual or potential life. But many of the simpler forms of life may undergo desiccation to such an extent as to arrest their vital manifestations and convert them into the semblance of not-living matter, and yet remain potentially alive. That is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life.
The properties of living matter are intimately related to temperature. Not only does exposure to heat sufficient to decompose protein matter destroy life, by demolishing the molecular structure upon which life depends; but all vital activity, all phenomena of nutritive growth, movement, and reproduction are possible only between certain limits of temperature. As the temperature approaches these limits the manifestations of life vanish, though they may be recovered by return to the normal conditions; but if it pass far beyond these limits, death takes place.
This much is clear; but it is not easy to say exactly what the limits of temperature are, as they appear to vary in part with the kind of living matter, and in part with the conditions of moisture which obtain along with the temperature. The conditions of life are so complex in the higher organisms, that the experimental investigation of this question can be satisfactorily attempted only in the lowest and simplest forms. It appears that, in the dry state, these are able to bear far greater extremes both of heat and cold than in the moist condition. Thus Pasteur found that the spores of fungi, when dry, could be exposed without destruction to a temperature of 120°-125° C. (248°-257° Fahr.), while the same spores, when moist, were all killed by exposure to 100° C. (212° Fahr.) On the other hand, Cagniard de La Tour found that dry yeast might be exposed to the extremely low temperature of solid carbonic acid (- 60° C. or -76° Fahr.) without being killed. In the moist state he found that it might be frozen and cooled to 5° C. (23° Fahr.), but that it was killed by lower temperatures. However, it is very desirable that these experiments should be repeated, for Cohn's careful observations on Bacteria show that, though they fall into a state of torpidity, and, like yeast, lose all their powers of exciting fermentation at, or near, the freezing point of water, they are not killed by exposure for five hours to a temperature below -10° C. (14° Fahr.), and, for some time, sinking to 18° C. ( - 0° 4 Fahr.) Specimens of Spirillum volutans, which had been cooled to this extent, began to move about some little time after the ice containing them thawed. But Cohn remarks that Euglenæ, which were frozen along with them, were all killed and disorganized, and that the same fate had befallen the higher Infusoria and Rotifera, with the exception of some encysted Vorticellæ, in which the rhythmical movements of the contractile vesicle showed that life was preserved.
Thus it would appear that the resistance of living matter to cold depends greatly on the special form of that matter, and that the limit of the Euglena, simple organism as it is, is much higher than that of the Bacterium.
Considerations of this kind throw some light upon the apparently anomalous conditions under which many of the lower plants, such as Protococcus and the Diatomaceæ, and some of the lower animals, such as the Radiolaria, are observed to flourish. Protococcus has been found, not only on the snows of great heights in temperate latitudes, but covering extensive areas of ice and snow in the Arctic regions, where it must be exposed to extremely low temperatures,in the latter case for many months together; while the Arctic and Antarctic seas swarm with Diatomacæ and Radiolaria. It is on the Diatomaceæ, as Hooker has well shown, that all surface life in these regions ultimately depends; and their enormous multitudes prove that their rate of multiplication is adequate to meet the demands made upon them, and is not seriously impeded by the low temperature of the waters, never much above the freezing point, in which they habitually live.
The maximum limit of heat which living matter can resist is no less variable than its minimum limit. Kühne found that marine Amba were killed when the temperature reached 35° C. (95° Fahr.), while this was not the case with fresh-water Amba, which survived a heat of 5°, or even l0°, C. higher. And Actinophrys Eichornii was not killed until the temperature rose to 44° or 45° C. Didymium serpula is killed at 35° C.; while another Myxomycete, Æthalium septicum, succumbs only at 40° C.
Cohn ("Untersuchungen über Bacterien," Beiträge sur Biologie der Pflanzen, Heft 2, 1872) has given the results of a series of experiments conducted with the view of ascertaining the temperature at which Bacteria are destroyed, when living in a fluid of definite chemical composition, and free from all such complications as must arise from the inequalities of physical condition when solid particles other than the Bacteria co-exist with them. The fluid employed contained 0.1 gramme potassium phosphate, 0.1 gr. crystallised magnesium sulphate, 0.1 gr. tri-basic calcium phosphate, and 0.2 gr. ammonium tartrate, dissolved in 20 cubic centimetres of distilled water. If to a certain quantity of this "normal fluid" a small proportion of water containing Bacteria was added, the multiplication of the Bacteria went on with rapidity, whether the mouth of the containing flask was open or hermetically closed. Hermetically-sealed flasks, containing portions of the normal fluid infected with Bacteria, were submerged in water heated to various temperatures, the flask being carefully shaken, without being raised out of the water, during its submergence.
The result was, that in those flasks which were thus subjected, for an hour, to a heat of 60o-62o C. (140°-143° Fahr.), the Bacteria underwent no development, and the fluid remained perfectly clear. On the other hand, in similar experiments in which the flasks were heated only to 40o or 50o C. (104°-132° Fahr.), the fluid became turbid, in consequence of the multiplication of the Bacteria, in the course of from two to three days.
Both in Kuhne's and in Cohn's experiments, which last have lately been confirmed and extended by Dr. Roberts of Manchester, it was noted that long exposure to a lower temperature than that which brings about immediate destruction of life, produces the same effect as short exposure to the latter temperatures. Thus, though all the Bacteria were killed, with certainty, in the normal fluid, by short exposure to temperature at or above 60° C. (140° Fahr.), Cohn observed that, when a flask containing infected normal fluid was heated to 50°-52° C. (120°-124° Fahr.) for only an hour, the consequent multiplication of the Bacteria was manifested much earlier, than in one which had been exposed for two hours to the same temperature.
It appears to be very generally held that the simpler vegetable organisms are deprived of life at temperatures as high as 60° C. (140° Fahr.), but Algæ have been found living in hot springs at much higher temperatures, namely, from 168° to 208° Fahr., for which latter surprising fact we have the high authority of Descloiseaux. It is no explanation of these phenomena, but only another mode of stating them, to say that these organisms have become "accustomed" to such temperatures. If this degree of heat were absolutely incompatible with the activity of living matter, the plants could no more resist it than they could become "accustomed" to being made red hot. Habit may modify subsidiary, but cannot affect fundamental, conditions.
Recent investigations point to the conclusion that the immediate cause of the arrest of vitality, in the first place, and of its destruction, in the second, is the coagulation of certain substances in the protoplasm, and that the latter contains various coagulable matters, which solidify at different temperatures. And it remains to be seen, how far the death of any form of living matter, at a given temperature, depends on the destruction of its fundamental substance at that heat, and how far death is brought about by the coagulation of merely accessory compounds.
It may be safely said of all those living things which are large enough to enable us to trust the evidence of microscopes, that they are heterogeneous optically, and that their different parts, and especially the surface layer, as contrasted with the interior, differ physically and chemically; while in most living things, mere heterogeneity is exchanged for a definite structure, whereby the body is distinguished into visibly different parts, which possess different powers or functions. Living things which present this visible structure are said to be organized; and so widely does organization obtain among living things, that organized and living are not unfrequently used as if they were terms of co-extensive applicability. This, however, is not exactly accurate, if it be thereby implied that all living things have a visible organization, as there are numerous forms of living matter of which it cannot properly be said that they possess either a definite structure or permanently specialized organs; though, doubtless, the simplest particle of living matter must possess a highly complex molecular structure, which is far beyond the reach of vision.
The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the material world, justify the separation of the biological sciences from all others. But it must not be supposed that the differences between living and not-living matter are such as to justify the assumption that the forces at work in the one are different from those which are to be met with in the other. Considered apart from the phenomena of consciousness, the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It may be convenient to use the terms "vitality" and "vital force" to denote the causes of certain great groups of natural operations, as we employ the names of "electricity" and "electrical force" to denote others; but it ceases to be proper to do so, if such a name implies the absurd assumption that "electricity" and "vitality" are entities playing the part of efficient causes of electrical or vital phenomena. A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital phenomena, depend,on the one hand, upon its construction, and, on the other, upon the energy supplied to it; and to speak of "vitality" as anything but the name of a series of operations is as if one should talk of the "horologity" of a clock.
Living matter, or protoplasm and the products of its metamorphosis, may be regarded under four aspects;
(1.) It has a certain external and internal form, the latter being more usually called structure;
(2.) It occupies a certain position in space and in time;
(3.) It is the subject of the operation of certain forces, in virtue of which it undergoes internal changes, modifies external objects, and is modified by them; and
(4.) Its form, place, and powers are the effects of certain causes.
In correspondence with these four aspects of the subject, biology is divisible into four chief subdivisionsI. Morphology; II. Distribution; III. Physiology; IV. Ætiology.
So far as living beings have a form and structure, they fall within the province of Anatomy and Histology, the latter being merely a name for that ultimate optical analysis of living structure which can be carried out only by the aid of the microscope.
And, in so far as the form and structure of any living being are not constant during the whole of its existence, but undergo A series of changes from the commencement of that existence to its end, living beings have a Development. The history of development is an account of the anatomy of a living being at the successive periods of its existence, and of the manner in which one anatomical state passes into the next
Finally, the systematic statement and generalization of the facts of Morphology, in such a manner as to arrange living beings in groups according to their degrees of likeness, is Taxonomy.
The study of Anatomy and Development has brought to light certain generalizations of wide applicability and great importance.
1. It has been said that the great majority of living beings present a very definite structure. Unassisted vision and ordinary dissection suffice to separate the body of any of the higher animals, or plants, into fabrics of different: sorts, which always present the same general arrangement in the same organism, but are combined in different ways in different organisms. The discrimination of these comparatively few fabrics, or tissues, of which organisms are composed, was the first step towards that ultimate analysis of visible structure which has become possible only by the recent perfection of microscopes and of methods of preparation.
Histology, which embodies the results of this analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is termed a cell; which cell, in its simplest condition, is merely a spheroidal mass of protoplasm, surrounded by a coat or sacthe cell-wallwhich contains cellulose. In the various tissues, these cells may undergo innumerable modifications of formthe protoplasm may become differentiated into a nucleus with its nucleolus, a primordial utricle, and a cavity filled with a watery fluid, and the cell-wall may be variously altered in composition or in structure, or may coalesce with others But, however extensive these changes may be, the fact that the tissues are made up of morphologically distinct units the cells remains patent. And, if any doubt could exist on the subject, it would be removed by the study of development, which proves that every plant commences its existence as a simple cell, identical in its
fundamental characters with the less modified of those cells of which the whole body is composed.
But it is not necessary to the morphological unit of the plant that it should be always provided with a cell-wall. Certain plants, such as Protococcus, spend longer or shorter periods of their existence in the condition of a mere spheroid of protoplasm, devoid of any cellulose wall, while, at other times, the protoplasmic body becomes enclosed within a cell-wall, fabricated by its superficial layer.
Therefore, just as the nucleus, the primordial utricle, and the central fluid are no essential constituents of the  morphological unit of the plant, but represent results of its metamorphosis, so the cell-wall is equally unessential; and either the term "cell" must acquire a merely technical significance as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconvenient to modify the sense of the word "cell."
The biological analysis of animal tissues has led to results and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifications which the cells undergo are so extensive, that the fact that the tissues are, as in plants, resolvable into an aggregation of morphological units, could never have been established without the aid of the study of development, which proves that the animal, no less than the plant, commences its existence as a simple cell, fundamentally identical with the less modified cells which are found in the tissues of the adult.
Though the nucleus is very constant among animal cells, it is not universally present; and among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may be, as in the lowest plants, devoid of a nucleus. In the animal, the cell-wall never has the character of a shut sac containing cellulose; and it is not a little difficult, in many cases, to say how much of the so-called "cell-wall" of the animal cell answers to the "primordial utricle" and how much to the proper "cellulose cell-wall" of the vegetable cell. But it is certain that in the animal, as in the plant, neither cell-wall nor nucleus are essential constituents of the cell, inasmuch as bodies which are unquestionably the equivalents of cellstrue morphological unitsare mere masses of protoplasm, devoid alike of cell-wall and nucleus.
For the whole living world, then, it results:that the morphological unitthe primary and fundamental form of life is merely an individual mass of protoplasm, in which no further structure is discernible; that independent living forms may present but little advance on this structure; and that all the higher forms of life are aggregates of such morphological units or celled variously modified.
Moreover, all that is at present known tends to the conclusion, that, in the complex aggregates of such units of which all the higher animals and plants consist, no cell has arisen otherwise than by becoming separated from the protoplasm of a pre-existing cell; whence the aphorism "Omnis cellula e cellula."
It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more superficial protoplasm of the cells; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole cell.
2. In the course of its development every cell proceeds from a condition in which it closely resembles every other cell, through a series of stages of gradually increasing divergence, until it reaches that condition in which it presents the characteristic features of the elements of a special tissue. The development of the cell is therefore a gradual progress from the general to the special state.
The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants my be, it begins its separate existence under the form of a nucleated cell. This, by division, becomes converted into an aggregate of nucleated cells: the parts of this aggregate, following different laws of growth and multiplication, give rise to the rudiments of the organs; and the parts of these rudiments again take on those modes of growth and multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure.
The development of the organism as a whole, therefore, repeats in principle the development of the cell. It is a progress from a general to a special form, resulting from the gradual differentiation of the primitively similar morphological units of which the body is composed.
Moreover, when the stages of development of two animals are compared, the number of these stages which are similar to one another is, as a general rule, proportional to the closeness of the resemblance of the adult forms; whence it follows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguishable. And this general rule holds for plants no less than for animals.
The broad principle, that the form in which the more complex living things commence their development is always the same, was first expressed by Harvey in his famous aphorism, "Omne vivum ex ovo," which was intended simply as a morphological generalization, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harvey's study of the development of the chick led him to promulgate that theory of "epigenesis," in which the doctrine that development is a progress from the general to the special is implicitly contained.
Caspar F. Wolff furnished further, and indeed conclusive, proof of the truth of the theory of epigenesis; but, unfortunately, the authority of Haller and the speculations of Bonnet led science astray, and it was reserved for Von Baer to put the nature of the process of development in its true light, and to formulate it in his famous law.
3. Development, then, is a process of differentiation by which the primitively similar parts of the living body become more and more unlike one another.
This process of differentiation may be effected in several ways.
(1.) The protoplasm of the germ may not undergo division and conversion into a cell aggregate; but various parts of its outer and inner substance may be metamorphosed directly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Algæ.
(2.) The germ may undergo division, and be converted into an aggregate of cells, which cells give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case
The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts and the differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become distinguishable may correspond on the two sides of the axis (bilateral symmetry), or may correspond along several lines parallel with the axis (radial symmetry).
The bilateral or radial symmetry of the body may be further complicated by its segmentation, or separation by divisions transverse to the axis, into parts, each of which corresponds with its predecessor or successor in the series.
In the segmented body, the segments may or may not give rise to symmetrically or asymetrically disposed processes, which are appendages, using that word in its most general sense.
And the highest degree of complication of structure, in both animals and plants, is attained by the body when it becomes divided into segments provided with appendages; when the segments not only become very different from one another, but some coalesce and lose their primitive distinctness; and when the appendages and the segments into  which they are subdivided similarly become differentiated and coalesce.
It is in virtue of such processes that the flowers of plants, and the heads and limbs of the Arthropoda and of the Vertebrata, among animals, attain their extraordinary diversity and complication of structure. A flower-bud is a segmented body or axis, with a certain number of whorls of appendages; and the perfect flower is the result of the gradual differentiation and confluence of these primitively similar segments and their appendages. The head of an insect or of a crustacean is, in like manner, composed of a number of segments, each with its pair of appendages, which by differentiation and confluence are converted into the feelers and variously modified oral appendages of the adult.
In some complex organisms, the process of differentiation, by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells giving rise to a particular portion of the adult organism. Thus the fertilized embryo cell in the archegonium of a fern divides into four cells, one of which gives rise to the rhizome of the young fern, another to its first rootlet, while the other two are converted into a placenta-like mass which remains embedded in the prothallus.
The structure of the stem of Chara depends upon the different properties of the cells, which are successively derived by transverse division from the apical cell. An inter-nodal cell, which elongates greatly, and does not divide, is succeeded by a nodal cell, which elongates but little, and becomes greatly subdivided; this by another inter-nodal cell; and so on in regular alternation. In the same way the structure of the stem, in all the higher plants, depends upon the laws which govern the manner of division and of metamorphosis of the apical cells, and of their continuation in the cambium layer.
In all animals which consist of cell-aggregates, the cells, of which the embryo is at first composed arrange themselves by the splitting, or by a process of invagination, of the blastoderm into two layers, the epiblast and the hypoblast, between which a third intermediate layer, the mesoblast, appears, and each layer gives rise to a definite group of organs in the adult. Thus, in the Vertebrata, the epiblast gives rise to the cerebro-spinal axis, and to the epidermis and its derivatives; the hypoblast, to the epithelium of the alimentary canal and its derivatives; and the mesoblast, to all the intermediate structures. The tendency of recent inquiry is to prove that the several layers of the germ evolve analogous organs in invertebrate animals, and to indicate the possibility. of tracing the several germ layers back to the blastomeres of the yelk, from the subdivision of which they proceed.
It is conceivable that all the forms of life should have presented about the same differentiation of structure, and should have differed from one another by superficial characters, each form passing by insensible gradations into those most like it. In this case Taxonomy, or the classification of morphological facts, would have had to confine itself to the formation of a serial arrangement representing the serial gradation of these forms in nature.
It is conceivable, again, that living beings should have differed as widely in structure as they actually do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations; in which case, again, classification could only effect the formation of seriesthe strict definition of groups would be as impossible as in the former case.
As a matter of fact, living beings differ enormously, not only in differentiation of structure, but in the modes in which that differentiation is brought about; and the intervals between extreme forms are not filled up in the existing world by complete series of gradations. Hence it arise that living beings are, to a great extent, susceptible of classification into groups, the members of each group resembling one another, and differing from all the rest, by certain definite peculiarities.
No two living beings are exactly alike, but it is a matter of observation that, among the endless diversities of living things, some constantly resemble one another so closely that it is impossible to draw any line of demarcation between them, while they differ only in such characters as are associated with sex. Such as thus closely resemble one another constitute a morphological species; while different morphological species are defined by constant characters which are not merely sexual.
The comparison of these lowest groups, or morphological species, with one another, shows that more or fewer of them possess some character or characters in commonsome feature in which they resemble one another and differ from all other species and the group or higher order thus formed is a genus. The generic groups thus constituted are susceptible of being arranged in a similar manner into groups of successively higher order, which are known as families, orders, classes, and the like.
The method pursued in the classification of living forms is, in fact, exactly the same as that followed by the maker of an index in working out the heads indexed. In an alphabetical arrangement, the classification may be truly termed a morphological one, the object being to put into close relation all those leading words which resemble one another in the arrangement of their letters, that is, in their form, and to keep apart those which differ in structure. Headings which begin with the same word, but differ otherwise, might be compared to genera with their species; the groups of words with the same first two syllables to families; those with identical first syllables to orders; and those with the same initial letter to classes. But there is this difference between the index and the Taxonomic arrangement of living forms, that in the former there is none but an arbitrary relation between the various classes, while in the latter the classes are similarly capable of co-ordination into larger and larger groups, until all are comprehended under the common definition of living beings.
The differences between "artificial" and "natural" classifications are differences in degree, and not in kind. In each ease the classification depends upon likeness; but in an artificial classification some prominent and easily observed feature is taken as the mark of resemblance or dissemblance; while, in a natural classification, the things classified are arranged according to the totality of their Morphological resemblances, and the features which are taken as the marks of groups are those which have been ascertained by observation to be the indications of many likenesses or unlikenesses And thus a natural classification is a great deal more than a mere index. It is a statement of the marks of similarity of organization; of the kinds of structures which, as a matter of experience, are found universally associated together; and, as such, it furnishes the whole foundation for those indications by which conclusions as to the nature of the whole of an animal are drawn from a knowledge of some part of it.
When a palæontologist argues from the characters of a bone or of a shell to the nature of the animal to which that bone or shell belonged, he is guided by the empirical morphological laws established by wide observation, that such a kind of bone or shell is associated with such and such structural features in the rest of the body, and no  others. And it is these empirical laws which are embodied and expressed in a natural classification.
Living beings occupy certain portions of the surface of the earth, inhabiting either the dry land or the fresh or salt waters, or being competent to maintain their existence in either. In any given locality, it is found that those different media are inhabited by different kinds of living beings; and that the same medium, at different heights in the air and at different depths in the water, has different living inhabitants.
Moreover, the living populations of localities which differ considerably in latitude, and hence in climate, always present considerable differences. But the converse proposition is not true; that is to say, localities which differ in longitude, even if they resemble one another in climate, often have very dissimilar faunæ and floræ.
It has been discovered by careful comparison of local faunæ and floræ that certain areas of the earth's surface are inhabited by groups of animals and plants which are not found elsewhere, and which thus characterize each of these areas. Such areas are termed Provinces of Distribution. There is no parity between these provinces in extent, nor in the physical configuration of their boundaries; and, in reference to existing conditions, nothing can appear to be more arbitrary and capricious than the distribution of living beings.
The study of distribution is not confined to the present order of nature; but, by the help of geology, the naturalist is enabled to obtain clear, though too fragmentary, evidence of the characters of the faunæ and floræ of antecedent epochs. The remains of organisms which are contained in the stratified rocks prove that, in any given part of the earth's surface, the living population of earlier epochs was different from that which now exists in the locality; and that, on the whole, the difference becomes greater the farther we go back in time. The organic remains which are found in the later Cainozoic deposits of any district are always closely allied to those now found in the province of distribution in which that locality is included; while in the older Cainozoic, the Mesozoic, and the Palæozoic strata, the fossils may be similar to creatures at present living in some other province, or may be altogether unlike any which now exist.
In any given locality, the succession of living forms may appear to be interrupted by numerous breaks associated species in each fossiliferous bed being quite distinct from those above and those below them. But the tendency of all palæontological investigation is to show that these breaks are only apparent, and arise from the incompleteness of the series of remains which happens to have been preserved in any given locality. As the area over which accurate geological investigations have been carried on extends, and as the fossiliferous rocks found in one locality fill up the gaps left in another, so do the abrupt demarcations .between the faunæ and floræ of successive epochs disappeara certain proportion of the genera and species of every period, great or small, being found to be continued for a longer or shorter time into the next succeeding period. It is evident, in fact, that the changes in the living population of the globe which have taken place during its history, have been effected, not by the sudden replacement of one set of living beings by another, but by a process of slow and gradual introduction of new species, accompanied by the extinction of the older forms.
It is a remarkable circumstance, that in all parts of the globe in which fossiliferous rocks have yet been examined, the successive terms of the series of living forms which have thus succeeded one another are analogous. The life of the Mesozoic epoch is everywhere characterized by the abundance of some groups of species of which no trace is to be found in either earlier or later formations; and the like is true of the Palæozoic epoch. Hence it follows, not only that there has been a succession of species, but that the general nature of that succession has been the same all over the globe; and it is on this ground that fossils are so important to the geologist as marks of the relative age of rocks.
The determination of the morphological relations of the species which have thus succeeded one another is a problem of profound importance and difficulty, the solution of which, however, is already clearly indicated. For, in several cases, it is possible to show that, in the same geographical area, a form A, which existed during a certain geological epoch, has been replaced by another form B. at a later period; and that this form B has been replaced, still later, by a third form C. When these forms, A, B. and C, are compared together they are found to be organized upon the same plan, and to be very similar even in most of the details of their structure; but B differs from A by a slight modification of some of its parts, which modification is carried to a still greater extent in C.
In other words, A, B. and C differ from one another in the same fashion as the earlier and later stages of the embryo of the same animals differ; and in successive epochs we have the group presenting that progressive specialization which characterizes the development of the individual. Clear evidence that this progressive specialization of structure has actually occurred has as yet been obtained in only a few cases (e.g., Equidæ, Crocodilia), and these are confined to the highest and most complicated forms of life; while it is demonstrable that, even as reckoned by geological time, the process must have been exceedingly slow.
Among the lower and less complicated forms the evidence of progressive modifications, furnished by comparison of the oldest with the latest forms, is slight, or absent; and some of these have certainty persisted, with very little change, from extremely ancient times to the present day. It is as important to recognize the fact that certain forms of life have thus persisted, as it is to admit that others have undergone progressive modification.
It has been said that the successive terms in the series of living forms are analogous in all parts of the globe. But the species which constitute the corresponding or homotaxic terms in the series, in different localities, are not identical And, though the imperfection of our knowledge at present precludes positive assertion, there is every reason to believe that geographical provinces have existed throughout the period during which organic remains furnish us with evidence of the existence of life. The wide distribution of certain Palæozoic forms does not militate against this view; for the recent investigations into the nature of the deep-sea fauna have shown that numerous Crustacea, Echinodermata, and other invertebrate animals have as wide a distribution now as their analogues possessed in the Silurian epoch.
Thus far living beings have been regarded merely as definite forms of matter, and Biology has presented no considerations of a different order from those which meet the student of Mineralogy. But living things are not only natural bodies, having a definite form and mode of structure, growth, and development. They are machines in action; and, under this aspect, the phenomena which they present have no parallel in the mineral world.
The actions of living matter are termed functions; and these functions, varied as they are, may be reduced to  three categories. They are either(1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other. Or (2), they are functions which subserve the process of reproduction, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed sustentative, the second generative, and the third correlative functions.
Of these three classes of functions the first two only can be said to be invariably present in living beings, all of which are nourished, grow, and multiply. But there are some forms of life, such as many Fungi, which are not known to possess any powers of changing their form; in which the protoplasm exhibits no movements, and reacts upon no stimulus; and in which any influence which the different parts of the body exert upon one another must be transmitted indirectly from molecule to molecule of the common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantly or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies; while, in all the higher animals, the different parts of the body are able to affect, and be affected by, one another, by means of a special tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle; and the organism is brought into relation with surrounding bodies by means of a third kind of special tissuethat of the sensory organs by means of which the forces exerted by surrounding bodies are transmuted into affections of nerve.
In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the starting point of their development. But the first process in that development is the division of the germ into a number of morphological units or blastomeres, which, eventually, give rise to cells; and as each of these possesses the same physiological functions as the germ itself, it follows that each morphological unit is also a physiological unit, and the multicellular mass is strictly a compound organism, made up of a multitude of physiologically independent cells. The physiological activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler constituents of that whole.
The morphological changes which the cells undergo in the course of the further development of the organism do not affect their individuality; and, notwithstanding the modification and confluence of its constituent cells, the adult organism, however complex, is still an aggregate of morphological units. Nor is it less an aggregate of physiological units, each of which retains its fundamental independence, though that independence becomes restricted in various ways.
Each cell, or that element of a tissue which proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or maintains a condition of equilibrium; but the most completely metamorphosed cells show no trace of the generative function, and many exhibit no correlative functions. On the other hand, those cells of the adult organism which are the unmetamorphosed derivatives of the germ, exhibit all the primary functions, not only nourishing themselves and growing, but multiplying, and frequently showing more or less marked movements.
Organs are parts of the body which perform particular functions. In strictness, perhaps, it is not quite right to speak of organs of sustentation or generation, each of these functions being necessarily performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are the apparatuses by which certain operations, subsidiary to sustentation and generation, are curried on.
Thus, in the case of the sustentative functions, all those organs may be said to contribute to this function which are concerned in bringing nutriment within reach of the ultimate cells, or in removing waste matter from them; while in the case of the generative function, all those organs contribute to the function which produce the cells from which germs are given off; or help in the evacuation, or fertilization, or development of these germs.
On the other hand, the correlative functions, so long as they are exerted by a simple undifferentiated morphological unit or cell, are of the simplest character, consisting of those modifications of position which can be effected by mere changes in the form or arrangement of the parts of the protoplasm, or of those prolongations of the protoplasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of its parts are brought about by the change of the form of certain tissues, the property of which is to shorten in one direction when exposed to certain stimuli. Such tissues are termed contractile; and, in their most fully developed condition, muscular. The stimulus by which this contraction is naturally brought about is a molecular change, either in the substance of the contractile tissue itself, or in some other part of the body; in which latter case, the motion which is set up in that part of the body must be propagated to the contractile tissue through the intermediate substance of the body. In plants there seems to be no question that parts which retain a hardly modified cellular structure may serve as channels for the transmission of this molecular motion; whether the same is true of animals is not certain. But, in all the more complex animals, a peculiar fibrous tissuenerve serves as the agent by which contractile tissue is affected by changes occurring elsewhere, and by which contractions thus initiated are coordinated and brought into harmonious combination. While the sustentative functions in the higher forms of life are still, as in the lower, fundamentally dependent upon the powers inherent in all the physiological units which make up the body, the correlative functions are, in the former, deputed to two sets of specially modified units, which constitute the muscular and the nervous tissues.
When the different forms of life are compared together as physiological machines, they are found to differ as machines of human construction do. In the lower forms, the mechanism, though perfectly well adapted to do the work for which it is required, is rough, simple, and weak; while, in the higher, it is finished, complicated, and powerful. Considered as machines, there is the same sort of difference between a polype and a horse as there is between a distaff and a spinning-jenny. In the progress from the lower to the higher organism, there is a gradual differentiation of organs and of functions. Each function is separated into many parts, which are severally entrusted to distinct organs. To use the striking phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physiological labour. And exactly the same process is observable in the development of any of the higher organisms; so that, physiologically, as well as morphologically, development is a progress from the general to the special.
Thus far, the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the influence of the "conditions of existence" thus determined constitutes a most important part of Physiology.
The sustentative functions, for example, can only be exerted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive "struggle for existence." The exercise of the correlative functions is influenced by similar conditions, and by the direct conflict with other organisms, which constitutes the active struggle for existence. And, finally, the generative functions are subject to extensive modifications, dependent partly upon what are commonly called external conditions, and partly upon wholly unknown agencies.
In the lowest forms of life, the only mode of generation at present known is the division of the body into two or more parts, each of which then grows to the size and assumes the form of its parent, and repeats the process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are severally competent to give rise to individual organisms of the same nature as that from which they arose.
In many of the lowest organisms the process is modified so far that, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as a bud which develops into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation are not confined to the simplest forms of life, however. On the contrary, both modes of multiplication are common not only among plants, but among animals of considerable complexity.
The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polypes by budding, are well-known examples of these modes of reproduction. In all these cases, the bud or the segment consists of a multitude of more or less metamorphosed cells. But, in other instances, a single cell detached from a mass of such undifferentiated cells contained in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segmentwhether the process is more akin to fission or to gemmation.
In all these cases the development of the new being from the detached germ takes place without the influence of other living matter. Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the way indicated above, ceases. At most, we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part; and, in the very highest animals, even this power vanishes in the adult; and, in most parts of the body, though the undifferentiated cells are capable of multiplication, their progeny grow, not into whole organisms like that of which they form a part, but into elements of the tissues.
Throughout almost the whole series of living beings, however, we find concurrently with the process of agamogenesis, or asexual generation, another method of generation, in which the development of the germ into an organism resembling the parent depends on an influence exerted by living matter different from the germ. This is gamogenesis, or sexual generation. Looking at the facts broadly, and without reference to many exceptions in detail, it may be
said that there is an inverse relation between agamogenetic and gamogenetic reproduction. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogenesis is absent. In many of the lower forms of life agamogenesis is the common and predominant mode of reproduction, while gamogenesis is exceptional; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis takes place exceptionally. In its simplest condition, which is termed "conjugation," sexual generation consists in the coalescence of two similar masses of protoplasmic matter, derived from different parts of the same organism, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism.
In the majority of cases, however, there is a marked morphological difference between the two factors in the process, and then one is called the male, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals it is a nucleated cell, to which a greater or less amount of nutritive material, constituting the food-yelk, may be added.
The male element, on the other hand, is relatively small. It may be conveyed to the female element by an outgrowth of the wall of its cell, which is short in many Algæ and Fungi, but becomes an immensely elongated tubular filament, in the case of the pollen cell of flowering plants. But, more commonly, the protoplasm of the male cells becomes converted into rods or filaments, which usually are in active vibratile movement, and sometimes are propelled by numerous cilia. Occasionally, however, as in many Nematoidea and Arthropoda, they are devoid of mobility.
The manner in which the contents of the pollen tube affect the embryo cell in flowering plants is unknown, as no perforations through which the contents of the pollen tube may pass, so as actually to mix with the substance of the embryo cell, have been discovered; and there is the same difficulty with respect to the conjugative processes of some of the Cryptogamia. But in the great majority of plants, and in all animals there can be no doubt that the substance of the male element actually mixes with that of the female, so that in all these cases the sexual process remains one of conjugation; and impregnation is the physical admixture of protoplasmic matter derived from two sources, which may be either different parts of the same organism, or different organisms.
The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (blastomeres), which may remain united as a single cell-aggregate, or some or all of which may become separate organisms. A larger or shorter period of rest, in many cases, intervenes between the act of impregnation and the commencement of the process of division.
As a general rule, the female cell which directly receives the influence of the male is that which undergoes division and eventual development into independent germs; but there are some plants, such as the Florideæ, in which this is not the case. In these the protoplasmic body of the trichogyne, which unites with the molecular spermatozoids, does not undergo division itself, but transmits some influence to adjacent cells, in virtue of which they become subdivided into independent germs or spores.
There is still much obscurity respecting the reproductive processes of the Infusoria; but, in the Vorticellidæ, it would appear that conjugation merely determines a condition of  the whole organism, which gives rise to the division of the endoplast or so-called nucleus, by which germs are thrown off; and if this be the case, the process would have some analogy to what takes place in the Florideæ.
On the other hand, the process of conjugation by which two distinct Diporpæ combine into that extraordinary double organism, the Diplozoon paradoxum, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individuals; and the same process takes place in a large number of the Infusoria, if what are supposed to be male sexual elements in them are really such.
The process of impregnation in the Florideæ is remarkably interesting, from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism other than the ovum, even in the highest animals and plants.
The nature of the influence exerted by the male element upon the female is wholly unknown. No morphological distinction can be drawn between those cells which are capable of reproducing the whole organism without impregnation, and those which need it, as is obvious from what happens in insects, where eggs which ordinarily require impregnation, exceptionally, as in many moths, or regularly, as in the case of the drones among bees, develop without impregnation. Even in the higher animals, such as the fowl, the earlier stages of division of the germ may tales place without impregnation.
In fact, generation may be regarded as a particular case of cell multiplication, and impregnation simply as one of the many conditions which may determine or affect that process. In the lowest organisms, the simple protoplasmic mass divides, and each part retains all the physiological properties of the whole, and consequently constitutes a germ whence the whole body can be reproduced. In more advanced organisms, each of the multitude of cells into which the embryo cell is converted at first, probably retains all, or nearly all, the physiological capabilities of the whole, and is capable of serving as a reproductive germ; but as division goes on, and many of the cells which result from division acquire special morphological and physiological properties, it seems not improbable that they, in proportion, lose their more general characters. In proportion, for example, as the tendency of a given cell to become a muscle cell or a cartilage cell is more marked and definite, it is readily conceivable that its primitive capacity to reproduce the whole organism should be reduced, though it might not be altogether abolished. If this view is well based, the power of reproducing the whole organism would be limited to those cells which had acquired no special tendencies, and consequently had retained all the powers of the primitive cell in which the organism commenced its existence. The more extensively diffused such cells were, the more generally might multiplication by budding or fission take place; the more localized, the more limited would be the parts of the organism in which such a process would take place. And even where such cells occurred, their development or non-development might be connected with conditions of nutrition. It depends on the nutriment supplied to the female larva of a bee whether it shall become a neuter or a sexually perfect female; and the sexual perfection of a large proportion of the internal parasites is similarly dependent upon their food, and perhaps on other conditions, such as the temperature of the medium in which they live. Thus the gradual disappearance of agamogenesis in the higher animals would be related with that increasing specialization of function which is their essential characteristic; and when it ceases to occur altogether, it may be supposed that no cells are left which retain unmodified the powers of the primitive embryo cell. The organism is like a society in which everyone is so engrossed by his special business that he has neither time nor inclination to marry.
Even the female elements in the highest organisms, little as they differ to all appearances from undifferentiated cells, and though they are directly derived from epithelial cells which have undergone very little modification from the condition of blastomeres, are incapable of full development unless they are subjected to the influence of the male element, which may, as Caspar Wolff suggested, be compared to a kind of nutriment. But it is a living nutriment, in some respects comparable to that which would be supplied to an animal kept alive by transfusion, and its molecules transfer to the impregnated embryo cell all the special characters of the organism to which it belonged.
The tendency of the germ to reproduce the characters of its immediate parents, combined, in the case of sexual generation, with the tendency to reproduce the characters of the male, is the source of the singular phenomena of hereditary transmission. No structural modification is so slight, and no functional peculiarity is so insignificant in either parent that it may not make its appearance in the offspring. But the transmission of parental peculiarities depends greatly upon the manner in which they have been acquired. Such as have arisen naturally, and have been hereditary through many antecedent generations, tend to appear in the progeny with great force; while artificial modifications, such, for example, as result from mutilation, are rarely, if ever, transmitted. Circumcision through innumerable ancestral generations does not appear to have reduced that rite to a mere formality, as it should have done, if the abbreviated prepuce had become hereditary in the descendants of Abraham; while modern lambs are born with long tails, notwithstanding the long-continued practice of cutting those of every generation short. And it remains to be seen whether the supposed hereditary transmission of the habit of retrieving among dogs is really what it seems at first sight to be; on the other side, Brown-Sèquard's case of the transmission of artificially induced epilepsy in guinea-pigs is undoubtedly very weighty.
Although the germ always tends to reproduce, directly or indirectly, the organism from which it is derived, the result of its development differs somewhat from the parent. Usually the amount of variation is insignificant; but it may be considerable, as in the so-called "sports;" and such variations, whether useful or useless, may be transmitted with great tenacity to the offspring of the subjects of them.
In many plants and animals which multiply both asexually and sexually, there is no definite relation between the agamogenetic and the gamogenetic phenomena. The organism may multiply asexually before, or after, or concurrently with, the occurrence of sexual generation.
But in a great many of the lower organisms, both animal and vegetable, the organism (A) which results from the impregnated germ produces offspring only agamogenetically. It thus gives rise to a series of independent organisms (B. B. B. . . .), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The process thus described is what has been termed the "alternation of generations" under its simplest form,for example, as it is exhibited by the Salpæ. In more complicated cases, the independent organisms which correspond with B may give rise agamogenetically to others (Bl), and these to others (B2), and 80 on (e.g., Aphis). But, however long the series, a final term appears which develops sexual organs, and reproduces A. The "alternation of generations" is, therefore, in strictness, an alternation of asexual with sexual generation, in which the products of the one process differ from those of the other.
 The Hydrozoa offer a complete series of gradations between those cases in which the term B is represented by a free, self-nourishing organism (e.g., Cyanæa), through those in which it is free but unable to feed itself (Caly cophoridæ), to those in which the sexual elements are developed in bodies which resemble free zooids, but are never detached, and are mere generative organs of the body on which they are developed (Cordylophora).
In the last case, the "individual" is the total products of the development of the impregnated embryo, all the parts of which remain in material continuity with one another The multiplication of mouths and stomachs in a Cordylophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede converts that Arthropod into a compound
animal The Cordylophora in a differentiation of a whole into many parts, and the use of any terminology which implies that it results from the coalescence of many parts into a whole is to be deprecated. In Cordylophora the generative organs are incapable of maintaining a separate existence; but in nearly allied Hydrozoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have undergone considerable changes of form. Morphologically, the swarm of Medusæ thus set free from a Hydrozoon are as much organs of the latter, as the multitudinous pinnules of a Comatula, with their genital glands, are organs of the Echinoderm. Morphologically, therefore, the equivalent of the individual Comatula is the Hydrozoic stock + all the Medusæ which proceed from it.
No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological individual; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical consequence, not only that all the polypes on a Cordylophoral tree are "feeding individuals," and all the genital sacs "generative individuals," while the stem must be a "stump individual," but that the eyes and legs of a lobster are "ocular" and "locomotive individuals." And this conception is not only somewhat more paradoxical than the other, but suggests a conception of the origin of the complexity of animal structure which is wholly inconsistent
Morphology, Distribution, and Physiology investigate and determine the facts of Biology. Ætiology has for its object the ascertainment of the causes of these facts, and the explanation of biological phenomena, by showing that they constitute particular cases of general physical laws. It is hardly needful to say that ætiology, as thus conceived, is in its infancy, and that the seething controversies, to which the attempt to found this branch of science made in the Origin of Species has given rise, cannot be dealt with in the limits of this article. At most, the general nature of the problems to be evolved, and the course of inquiry needful for their solution, may be indicated.
In any investigation into the causes of the phenomena of life the first question which arises is, whether we have any knowledge, and if so, what knowledge, of the origin of living matter?
In the case of all conspicuous and easily-studied organisms, it has been obvious, since the study of nature began, that living beings arise by generation from living beings of a like kind; but before the latter part of the 17th century, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of not-living, and especially of putrefying dead matter, by what was then termed generatio æquivoca or spontanea, and is now called abiogenesis. Redi showed that the general belief was erroneous in a multitude of instances; Spallanzani added largely to the list; while the investigations of the scientific helminthologists of the present century have eliminated a further category of cases in which it was possible to doubt the applicability of the rule "omni vivum e vivo" to the more complex organisms which constitute the present fauna and flora of the earth .Even the most extravagant supporters of Biogenesis at the present day do not pretend that organisms of higher rank that the lowest Fungi and Protozoa are produced otherwise than by generation from pre-existing organisms. But it is pretended that Bacteria, Tortulæ, certain Fungi, and "Monads" are developed under conditions which render it impossible that these organisms should have proceeded directly from living matter.
The experimental evidence adduced in favour of this proposition is always of one kind, and the reasoning on which the conclusion that abiogenesis occurs is based may be stated in the following form:
All living matter is killed by being heated to n degrees.
The contents of the closed vessel A have been heated to n degrees.
Therefore, all living matter which may have existed therein has been killed.
But living Bacteria, &c., have appeared in these contents subsequently to their being heated.
Therefore, they have been formed abiogenetically.
No objection can be taken to the logical form of this reasoning, but it is obvious that its applicability to any particular case depends entirely upon the validity, in that case, of the first and second propositions
Suppose a fluid to be full of Bacteria in active motion, what evidence have we that they are killed when that fluid is heated to n degrees? There is but one kind of conclusive evidence, namely, that from that time forth no living Bacteria make their appearance in the liquid, supposing it to be properly protected from the intrusion of fresh Bacteria. The only other evidence, that, for example, which may be furnished by the cessation of the motion of the Bacteria, and such slight changes as our microscopes permit us to observe in their optical characters, is simply presumptive evidence of death, and no more conclusive than the stillness and paleness of a man in a swoon are proof that he is dead. And the caution is the more necessary in the case of Bacteria, since many of them naturally pass a considerable part of their existence in a condition in which they show no marks of life whatever save growth and multiplication.
If indeed it could be proved that, in cases which are not open to doubt, living matter is always and invariably killed at precisely the same temperature, there might be some ground for the assumption, that, in those which are obscure, death must take place under the same circumstances. But what are the facts? It has been pointed out at the commencement of this article, that the range of high temperatures between the lowest, at which some living things are certainly killed, and the highest, at which others certainly live, is rather more than 100° Fahr., that is to say, between 104° Fahr. and 208° Fahr. It makes no sort of difference to the argument how living beings have come to be able to bear such a temperature as the last mentioned; the fact that they do so is sufficient to prove that, under certain conditions, such a temperature is not sufficient to destroy life.
Thus it appears that there is no ground for the assumption that all living matter is killed at some given temperature between 104° and 208° Fahr.
 But, further, there is very strong reason for believing that the influence of temperature on life is greatly modified, first, by the nature of the medium in which organisms are placed, and, secondly, to the length of time during which any given temperature is kept up.
On this point recent experiments made by Dr. Roberts of Manchester are of great importance. He found, for example, as every other careful experimenter has done, that ordinary infusion of hay boiled for a few minutes was sterilized, that is to say, no development of Bacteria took place in it, however long it might be kept; while if the infusion was rendered alkaline with ammonia or liquor potages, it was not sterilized except after an exposure to the heat of boiling water for more than an hour. Sometimes it became productive after two hours, and once after three hours of such exposure. Is it to be imagined that, in the case of the alkalized hay infusion, the heat applied really killed the Bacteria which existed in the infusion, and that Bacteria of identically the same kind were generated afresh out of the dead matter, or is it more probable that the powers of resistance of the Bacteria to heat were simply increased by the alkalinity of the infusion? The statement of the questions surely render it unnecessary to answer them
Dr Roberts further proves that there are two factors in the induction of sterilization, the degree of heat on the one hand, and the duration of its application on the other. A longer exposure to a lower temperature was equivalent to a shorter exposure to a higher temperature. "For example, speaking roughly, an exposure of an hour and a half to a heat of 212° Fahr. appeared to be equivalent to an exposure for fifteen minutes to a heat of 228° Fahr."1
It is hard to conceive what explanation can be offered of this fact, except that, under the conditions of the experiment, the organisms were either all affected by the first incidence of the heat in such a way as only to arrest some of their vital functions, and to leave a potentiality of life in them, such as exists in some kinds of dried living matter; or that they individually differed very much in their powers of resistance, and that some were able to withstand heat much longer than others.
Under these circumstances it will be evident, that no experimental evidence that a liquid may be heated to n degrees, and yet subsequently give rise to living organisms, is of the smallest value as proof that abiogenesis has taken place, and for two reasons:Firstly, there is no proof that organisms of the kind in question are dead, except their permanent incapacity to grow and reproduce their kind; and secondly, since we know that conditions may largely modify the power of resistance of such organisms to heat, it is far more probable that such conditions existed in the experiment in question, than that the organisms were generated afresh out of dead matter.
Not only is the kind of evidence adduced in favour of Biogenesis logically insufficient to furnish proof of its occurrence, but it may be stated as a well-based induction, that the more careful the investigator, and the more complete his mastery over the endless practical difficulties which surround experimentation on this subject, the more certain are his experiments to give a negative result; while positive results are no less sure to crown the efforts of the clumsy and the careless.
It is argued that a belief in abiogenesis is a necessary corollary from the doctrine of Evolution. This may be true, of the occurrence of abiogenesis at some time; but if the present day, or any recorded epoch of geological time, be in question, the exact contrary holds good. If all living beings have been evolved from pre-existing forms of life, it is, enough that a single particle of living protoplasm should once have appeared on the globe, as the result of no matter what agency. In the eye of a consistent evolutionist any further independent formation of protoplasm would be sheer waste.
The production of living matter since the time of its first appearance, only by way of biogenesis, implies that the specific forms of the lower kinds of life have undergone but little change in the course of geological time, and this is said to be inconsistent with the doctrine of evolution. But, in the first place, the fact is not inconsistent with the doctrine of evolution properly understood, that doctrine being perfectly consistent with either the progression, the retrogression, or the stationary condition of any particular species for indefinite periods of time; and secondly, if it were, it would be so much the worse for the doctrine of evolution, inasmuch as it is unquestionably true, that certain, even highly organized, forms of life have persisted without any sensible change for very long periods. The Terebratula psittacea of the present day, for example, is not distinguishable from that of the Cretaceous epoch, while the highly organized Teleostean fish, Beryx, of the Chalk differed only in minute specific characters from that which now lives. Is it seriously suggested that the existing Terebratulæ and Beryces are not the lineal descendants of their Cretaceous ancestors, but that their modern representatives have been independently developed from primordial germs in the interval? But if this is too fantastic a suggestion for grave consideration, why are we to believe that the Globigerinæ of the present day are not lineally descended from the Cretaceous forms? And if their unchanged generations have succeeded one another for all the enormous time represented by the deposition of the Chalk and that of the Tertiary and Quaternary deposits, what difficulty is there in supposing that they may not have persisted unchanged for a greatly longer period?
The fact is, that at the present moment there is not a shadow of trustworthy direct evidence that abiogenesis does take place, or has taken place, within the period during which the existence of life on the globe is recorded. But it need hardly be pointed out, that the fact does not in the slightest degree interfere with any conclusion that may be arrived at deductively from other considerations that, at some time or other, abiogenesis must have taken place.
If the hypothesis of evolution is true, living matter must have arisen from not-living matter; for by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it,2 life being entirely incompatible with the gaseous state. But living matter once originated, there is no necessity for another origination, since the hypothesis postulates the unlimited, though perhaps not indefinite, modifiability of such matter.
Of the causes which have led to the origination of living matter, then, it may be said that we know absolutely of nothing. But postulating the existence of living matter endowed with that power of hereditary transmission, and with that tendency to vary which is found in all such matter, Mr During has shown good reasons for believing that the interaction between living matter and surrounding conditions, which results in the survival of the fittest, is sufficient to account for the gradual evolution of plants and animals from their simplest to their most complicated forms, and for the known phenomena of Morphology, Physiology, and Distribution.
 Mr. Darwin has further endeavoured to give a physical explanation of hereditary transmission by his hypothesis of Pangenesis; while he seeks for the principal, if not the only, cause of variation in the influence of changing conditions.
It is on this point that the chief divergence exists among those who accept the doctrine of Evolution in its general outlines. Three views may be taken of the causes of variation:
a. In virtue of its molecular structure, the organism may tend to vary. This variability may either be indefinite, or may be limited to certain directions by intrinsic conditions. In the former case, the result of the struggle for existence would be the survival of the fittest among an indefinite number of varieties; in the latter case, it would be the survival of the fittest among a certain set of varieties, the nature and number of which would be predetermined by the molecular structure of the organism
b. The organism may have no intrinsic tendency to vary, but variation may be brought about by the influence of conditions external to it. And in this case also, the variability induced may be either indefinite or defined by intrinsic limitation.
c. The two former cases may be combined, and variation may to some extent depend upon intrinsic, and to some extent upon extrinsic, conditions.
At present it can hardly be said that such evidence as would justify the positive adoption of any one of these views exists.
If all living beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish, and then a tailed amphibian, provided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifications whereby some ancient fish became an urodele amphibian; and the urodele amphibian became an anurous amphibian. In fact, the development of the embryo is a recapitulation of the ancestral history of the species.
If this be so, it follows that the development of any organism should furnish the key to its ancestral history; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their development has given rise to a special branch of biological speculation, termed phylogeny.
In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation.
The only perfectly safe foundation for the doctrine of Evolution lies in the historical, or rather archæological, evidence that particular organisms have arisen by the gradual modification of their predecessors, which is furnished by fossil remains. That evidence is daily increasing
in amount and in weight; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced from the facts of embryology alone, may be satisfactorily tested.
1 Proceedings of the Royal Society, No. 152, p. 290. 2 It makes no difference if we adopt Sir W. Thomson's hypothesis and suppose that the germs of living things have been transported to our globe from some other, seeing that there is as much reason for supposing that all stellar and planetary components of the universe are or have been gaseous, as that the earth has passed through this stage. Bibliography.Haeckel, Generelle Morphologie; H. Spencer, Principles of Biology.
1 Proceedings of the Royal Society, No. 152, p. 290.
2 It makes no difference if we adopt Sir W. Thomson's hypothesis and suppose that the germs of living things have been transported to our globe from some other, seeing that there is as much reason for supposing that all stellar and planetary components of the universe are or have been gaseous, as that the earth has passed through this stage.
Bibliography.Haeckel, Generelle Morphologie; H. Spencer, Principles of Biology.
C. Blinderman & D. Joyce