Biology

Biology (Gr. bios, 'life'; logos, 'discourse'). This term has been occasionally abused in English popular writing, more especially in the absurd word electro-biology, which at one time threatened to take root in popular usage (see ANIMAL MAGNETISM), and has even by some scientific writers been confused with general physiology, or a special province of it. Yet the established and only legitimate meaning of biology is its literal one, that of the science of life—i.e. the science which seeks to classify and generalise the vast and varied multitude of phenomena presented by and peculiar to the living world. In the following outline the science and its general relations may be conveniently discussed under seven heads.

1. Origin and Rise of Biology.—Science being simply a systematised development of our common experience, it is with no more than that simplest knowledge of living beings possessed by the savage or the child that the future biologist sets out. These already keenly observe the forms of life around them: the sheep and goat, the dog and cat, the fowl and sparrow, the trout and salmon, are all clearly recognised and named; and similarly with plants. Classification, too, naturally arises; the birds, beasts, and fishes become more and more clearly defined; and even the residual mob of 'creeping things' discloses possibilities of orderly arrangement. Cedar and oak obtain the common name of trees, briar and bramble of bushes, balm and hyssop of herbs; but beyond this level it is not easy to pass. He is therefore 'the best botanist who knows most plants,' the greatest naturalist who can recognise the appearance and recall the names of the greatest number of living things, or at any rate can remember most about each of them. This fundamental but as yet scarcely scientific knowledge comes to be recorded in a literature of natural history which varies indeed in accuracy and fullness, yet remains unchanged in spirit and purpose whether we open the pages of Pliny, Gesner, or Buffon. White's Natural History of Selborne may be taken as the classic English example of this nascent phase of biology; and its wide acceptance is not hard to understand, since at this level of simple but comprehensive interest in living beings, every one is by nature, however unconsciously, something of a biologist. But the habit of observation insensibly develops an increased perception of likenesses and differences; collections too arise; the power and need of classification develop and cultivate each other beyond the popular level, and a community of naturalists is thus insensibly formed. These divide, in the first place, into zoologists and botanists, among whom minor circles of specialists occupied with definite groups appear in turn. While this division of labour is apparent, its result is always giving us more and more definite general ideas of the groups studied, and this process of generalisation is summed up from time to time by a mind which unites the old breadth of interest with the newer depth of knowledge. Classifications are continually approximating towards completeness; our masses of knowledge respecting the plant and the animal world are at last brought together as botany and zoology, and the final advance is made through seeking to unite the generalisations of the two. Just as ostrich and swallow, shark and salmon, become united into the common logical genera of bird or fish, so inevitably do plant and animal become united into the general conception of organism. Their details of habit and processes of life come to be viewed in general terms of environment and function. Unity becomes more and more discerned under the manifold variety of these, and the conception of a united and general biology at last lies clear before us. It is, in fact, with the naturalist as with the chemist or mineralogist: the latter has his interest at first awakened by the variety of glistening stones, but soon finds himself eager to interpret and systematise the actual varieties of crystalline form in terms of a few simple ideal ones, with modifications in relation to particular circumstances; while the former passes from the empirical description of the experimental details of the science to the interpretation of these by reference to the properties of chemical elements and the laws of their molecular interaction and arrangement. Similarly, the naturalist passes from empirically recording his observations of living nature or the treasuring of its defunct spoils, to analyse and unify the general problems of life; and thus becomes a biologist proper. It may be well clearly to sum this historical advance diagrammatically :

From this diagram the striking contrast, yet continuous gradation, between two such familiar classics as White's Sclborne and Spencer's Principles of Biology may be clearly understood, and by tracing the steps of its correction and development (see BOTANY and ZOOLOGY), the history of scientific advance might be recorded, and even the field of each investigator marked out. Since these details will be outlined under BOTANY and ZOOLOGY, it suffices here to point out that progress has lain in continuously uniting these extremes, so that the crude empiricism which at first absorbed the more concrete students, and the vague and useless metaphysics which were wont to discredit the abstract ones, are being steadily replaced by a compact body of generalisation, along which the specialist at any point may safely ascend to the widest theories of the science, and by which the most abstract thinker may similarly return to lay clear hold of the minutest details of any order of empirical fact. Hence we can clearly understand the way in which the conception of biology was actually first formulated at the beginning of the century, independently and simultaneously by Lamarck and Treviranus (1802); the former rising from his early botanical studies and his Histoire Naturelle des Animaux sans Vertèbres to the elevated conceptions of his Philosophie Zoologique, which reached general principles as the outcome of detailed studies; while the more abstract mind of the German Treviranus occupied itself with details essentially as interpreted by the help of general principles.

The conception of biology, it will be noted, has thus come to be used with two different contents: in speaking of the principles of biology, we refer to the body of generalisations common to plants and animals; while in speaking of the science of biology, we include the entire body of botany and zoology, with all their concrete sub-sciences. Yet there is here no discord, and we may conveniently accept the definition of Charles Robin: 'Biology is the science which has for its object the study of organic beings, and for its end the knowledge of the laws of their organisation and activity.'

2. Subordinate Provinces of Biology.—The results of this natural history study of plants and animals, which seeks above all things to know the actual forms of life, are of course in the first place to be sought in the numerous separate articles devoted to particular animals and plants; next, in the articles devoted to separate families, natural orders, classes, and sub-kingsdoms, and finally under BOTANY and ZOOLOGY; while the facts relating to their distribution upon the earth's surface, in present and in past time, are outlined under GEOGRAPHICAL DISTRIBUTION and PALEONTOLOGY respectively. Underlying this empirical view of plants and animals, yet essential to the classification which can alone give this any intelligibility or completeness, lies a detailed analysis of each of these multitudinous forms, which we call anatomy. This involves comparison, and comparative anatomy arises; but as we come to discern the marvellous unity of type which underlies all apparent differences, and as we (e.g.) decompose all flowers into variously arranged leaf-whorls, or fancy that we can reduce all skulls to vertebrae, or again analyse the organism into organ, tissue, or cell, the conception arises of what was at first called transcendental or philosophical anatomy, but which later became known as morphology. This term has also been extended, and on the whole with advantage, to include not only these widest generalisations of unity in organic forms, but all the details as well, in fact all the static aspects of organisms as distinguished from their dynamic or vital ones, which we similarly group under physiology. In the same way the crystallographer is, as it were, the pure morphologist, but with him we can scarcely avoid including the descriptive mineralogist as well.

But while the morphologist deals primarily with the anatomical facts presented by the organism before him at a given moment, he can only fully understand the origin of these and still more their resemblances and differences by reference to its form at earlier and earlier past times; and in the endeavour to reach this fuller conception of form no longer viewed at rest, but as a result and phase of change, we constitute the sub-science of embryology. See ANATOMY, MORPHOLOGY, EMBRYOLOGY.

Yet our biological studies are even now not biological in the most literal sense, for the study of mere forms can take no note of the life which possessed or produced them. The morphologist's dealings are with fossil and corpse; his botanic garden is a herbarium, his appliances the scalpel and lens; whereas it is the harder task of his physiological colleague, instead of simply describing the machinery of life, to comprehend it in action. He thus of course requires an acquaintance with the results of the study of forms, but only for the sake of applying, and if possible interpreting these. In a word, his problems are no longer those of statics, but of dynamics. These dynamic or physiological problems in the first place concern the individual organism at a given time both as a whole and analysed; we study its internal and external relations—its respiration and nutrition in all cases, its sensation and movement if such there be. But the conception of phases of function at successive times necessarily also presents itself; and the more complex problems of growth and adaptation, of variation and of disease, thus arise; while when we pass from the study of the organism alone to its relations to the species, the processes of reproduction and the marvels of heredity appear for elucidation (see ENVIRONMENT, HEREDITY, PATHOLOGY, PHYSIOLOGY, REPRODUCTION, VARIATION). And in all these cases, as we shall see below, these problems may be attacked at different levels—i.e. in terms of organ, tissue, cell, or protoplasm.

3. History of Morphology and Physiology.—The progress of our concrete knowledge of plants and animals is respectively outlined under BOTANY and ZOOLOGY, and even from these articles it will be sufficiently obvious that these two developments are not mere isolated streams of discovery, but have a deeper than biographical interest. Our historic inquiry must now seek to fuse these two detailed chronicles of botanical and zoological advance into a broad and general sketch. This should show how the whole development of modern biology, despite its multitudes of past and present workers, and its apparently overpowering wealth and variety of detail, lies upon a few essential lines of investigation, which have been opened up by a magistral succession of original thinkers, and simply extended and elaborated by their respective schools. For were this grasp of general principle once obtained, it is evident that we should be in possession, not merely of an additional fragment of more or less interesting historical information, but of a well-twined and trustworthy clue which should at once guide us through the teeming maze of organic nature, and among the corresponding recesses of the vast and increasing library of biologic science. (a) Morphology.—Starting then from those chaotic accumulations of fact and blunder, myth and fancy, of practical usefulness and traditional quackery, which characterise the medieval encyclopædias of natural history, we find their inevitable criticism and replacement at length fairly undertaken in the latter half of the 17th century by the labours of John Ray and his minor contemporaries. Yet the period of general natural history was not to end for a century, for it is in Buffon's wide and brilliant survey of all that was known of animal life—the famous Histoire Naturelle Générale—that the ancient school found at once its highest outcome, its monument and grave. The modern period, then, essentially opens with Linnæus, whose importance depends upon his having brought to bear upon the confused accumulations of the past the critical and analytic skill, the accuracy and descriptive power, and the classificatory resources of existing logic, which had hitherto been dragging on a merely academic or at most controversial existence, but left almost devoid of concrete or constructive applications. The importance of the order introduced by his binomial nomenclature, his establishment of the categories of species and genus, order and class, his precision of descriptive terminology and the like, can hardly be overrated; while his definite concentration for classificatory purposes upon the precise form of the organism as distinguished from its mere general appearance and mode of life, laid the basis of the future science of pure morphology. Despite the opposition of Buffon, whose thoroughly synthetic attitude and ardent enthusiasm of living nature were alike naturally repugnant to the cold analysis of the new Linnæan school, this necessarily triumphed, indeed too completely. The orderly and exhaustive catalogue of natural forms begun by Linnæus was ardently taken up by his pupils, and handed on to his innumerable intellectual heirs; and it is still in progress, as every systematic monograph, such as those of the Challenger Expedition, still testifies. Each new species described means in fact a new leaf added to his Systema Naturæ, and the original work is thus, as it were, under continual revision and extension by a succession of sectional sub-editors. Yet, although invaluable as an index to the museum of nature, and indispensable as a catalogue for the future library of the science, the preparation of this systematic description of species gradually enslaved its labourers, and blinded them to all the other problems presented by the living beings which came under their notice. But although this disastrous tendency remains even now too common, the required deepening of the science was soon fairly begun by Jussieu, who passed from the description of external forms alone, and the artificial and provisional grouping of species by their superficial characters, to the establishment of large natural alliances upon a basis of comparative anatomy. He thus not only founded the 'natural system' in botany, but suggested to Cuvier that re-examination of the animal world upon similar lines which became his life-work. But this supplementing of superficial description by detailed anatomical and comparative research has not only given us the classic Règne Animal of Cuvier, but its continuation by an entire school—the comparative anatomists. Of these Meckel, Müller, and Gegenbaur in Germany, Milne-Edwards and Lacaze-Duthiers in France, Owen and Huxley in

England, will be remembered among the most historic veterans.

The next step, that of penetration below the study of organs, is due to Bichat, who analysed the body into a series of simple tissues—muscular, nervous, glandular, connective, &c., with definite structural characters. Here, then, a new movement—the histological—found its beginning, and thus under Bichat's Anatomie Générale we may group not only the labours of his immediate successors, but all those researches on embryonic layers and tissue structure which have of late formed an increasing proportion of recent literature.

Histology had not, however, found in the study of tissues its ultimate basis. The microscopic studies of the botanist Schleiden led him, in 1838, to the conclusion that all plant structures were built up of separate unit-masses or cells, a generalisation immediately extended to animal structures by Schwann, who published in the following year his famous Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals, as the English translation (Sydenham Society, 1847) has it. The tissue being in this way analysed into a cell-aggregate, that elaborate and detailed study of cellular forms and structures, development and modifications, which still mainly occupies the histologist, was thus fairly begun.

Finally, the description of the essential living substance of some of the lowest animal cells by Dujardin, and the similar account of vegetable cell-substance by Von Moll, have directed attention from the cell to the all-important protoplasm of which it is composed. Here, of course, opens a new series of investigations, in which a basis for the study of organic structure is sought in the investigation of protoplasm. No deeper analysis is possible, without passing out of morphology altogether into chemistry and physics; and we are thus warranted in regarding the preceding five great categories of morphological inquiry as exhaustive, although progress within each of these is still far from ended. It must not be forgotten, indeed, that especially since the establishment of the last two of these categories (cell and protoplasm) we have had a new light cast upon all five through the study of development, with which names like those of Von Baer, Kölliker, and others, are so imperishably associated. Thus we can now trace not only the origin and differentiation of the cell itself, but that of the tissue, the organ, and the organism, from the ovum or primeval cell. Recognising, however, this general indebtedness to the embryologist for elucidating, under the actual facts of being, their mode of becoming, we may still classify the labours of morphologists as being essentially continuations of the five fundamental classics above mentioned. (b) Physiology.—The early physiology was a matter of very vague and often mystical interpretation of the processes of the body, as phrases like those of 'animal spirits' and 'vital spirits' still survive to show. At best it was broadly little more than a superficial account of habit and temperament, which, although corresponding to the observation of general form later inaugurated by Linnæus, had yet little of the order and definiteness of this. The progress of anatomical research could not, however, fail to suggest that many vital processes were associated with definite organs or systems of organs, and the analytic study of physiology may be thus regarded as beginning with Harvey's memorable demonstration of the circulation of the blood. The study of the functions of each organ, viewed as a whole, long continued to form the sole problem of physiology. The illustrious Bichat was, however, no less a physiological than a morphological thinker; and in the physiological portion of his

Anatomie Générale, function was referred from the organ as a whole to the fundamental properties of its essential tissue. He thus not only deepened both morphology and physiology by a new analysis, but showed them to coincide; and in this way we come to understand how he so early clearly reached the conception of a unified science of biology.

With the advent of the cell theory, function, which had thus been referred from organ to tissue, had of course next to receive a yet deeper interpretation in terms of the constituent cells of these. Cellular physiology and pathology were clearly foreseen by Goodsir, and soon developed by Virchow and his school. Yet the interpretation of function in relation to structure, which had been successively undertaken at each deepening level, began even at this one to break down. Attention came to be directed to the nature of protoplasm, and physiology, as Foster expresses it, thus began to undergo 'a change of front.' For the physiologist has now to view the problems of function of organ, tissue and cell alike, as outcomes or accompaniments of the destructive and constructive molecular changes (metabolism) of its protoplasm, of the physical and chemical processes of its waste and repair, its katabolism and anabolism respectively. This ultimate school of physiology may be headed by the works of the late illustrious Claude Bernard.

It is a fact of no little significance that these two independently deepened lines of research, morphological and physiological, are accurately parallel, stage for stage; not a little remarkable that the two sub-sciences of morphology and physiology should have independently undergone a precisely similar evolution. The result of our survey may in fact be conveniently illustrated by means of a couple of similar bookcases—one for morphology, the other for physiology—the shelves of each being allotted, in descending order, to the literature of the structural and functional aspects respectively of organism, organ, tissue, cell, and protoplasm, and each shelf being headed by its initial classic. To this biological pentateuch, of morphology and physiology respectively, each set of subsequent researches must be simply regarded as commentary or appendix. For since not qualitatively distinct, the originality of these is simply of a quantitative order. And if we imagine these cases, instead of being placed side by side as in the diagram, to be situated back to back, we have a yet clearer image of the completeness of their parallelism, as well as of the way in which they have independently been filled by workers approaching from quite different sides.

(c) Evolution.—Where, however, it may be asked, is the position on such a scheme of the literature of evolution? The reply of course is, that like the conception of individual development of form and function, that of racial development does not lie upon any one level, but equally below them all. The evolutionist, like the embryologist, of course considers form and function in process of continuous change, while the morphologist or physiologist views them as constant at any given moment. Hence the diagrammatic representation of our evolutionary conceptions would lie in another plane; its literature might indeed be placed on the same shelves, but behind the volumes occupying them. The history of the doctrine of evolution, like that of embryology, may therefore more conveniently be treated separately (see EMBRYOLOGY, EVOLUTION), while the subject of distribution, not being a department of pure biology at all, but rather of geology and physical geography, does not enter into the present discussion (see DISTRIBUTION).

4. Nature and Origin of Life.—It is by this time sufficiently clear that the biologist is solely concerned with the positive facts of living nature, and is seeking only verifiable generalisations. He accepts the relativity of all scientific knowledge, and has long been wholly freed from alchemist-like dreams of reaching any 'vital essence' or ultimate secret of life other than that of a classified and unified account of its phenomena. The history of speculation as to the ultimate nature of life may therefore be conveniently relegated to a separate article (see LIFE), in which the various attempts to define it will be found: for actual biological purposes, the life of an organism is simply the sum of its functions, internal and external; nor need any other working conception be desired, if we agree that the ideal of organic life, that of a maximum of healthy function, implies the harmonised yet progressive adaptation of function and environment. Similarly, the history of speculation as to the unknown origin of life is separately discussed (see SPONTANEOUS GENERATION); while such actual knowledge as we possess respecting the structural and functional properties and characteristics of living matter is summarised under PROTOPLASM.

5. Place of Biology among the Sciences.—While no special account of the logic of biology is required, save that its study demands 'observation and experiment, comparison, classification, and historic filiation'—in short, all the powers of the scientific intelligence—its relation to other studies is a matter of more serious dispute. The essential problems, briefly stated, are: (1) How far and in what way the phenomena of biology are interpretable by and related to those of chemistry and physics; (2) How physiological phenomena are related to mental ones. The reply which is offered to these questions by many biologists may be best stated in two quotations from Huxley: '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. . . . A mass of 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 "horology" of a clock.' Again, 'the biological sciences are those which deal with the phenomena of 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 associated with physiology, and the phases of social life exhibited by animals other than man fall strictly within the province of the biologist.'

From these positions the transition is easy to the dogmatic materialism of writers who look forward to completely reducing functional and psychical processes to chemico-physical ones. But it has long been pointed out that the functional states and changes which we call physiological, and the mental states and changes which we know as psychological, have each their own definiteness and continuity. Motions are continuously traceable to antecedent motions, which may be termed their causes; nor is there any hiatus at which 'vitality' or the like can be inserted; similarly, however, changes in mentality are preceded by antecedent changes of mentality; which are equally entitled to be termed their causes. A comparison of the two streams shows indeed a definite parallelism between them; so that we constantly interpret one by the other. Feature, voice, and gesture are thus familiarly regarded as the 'expression' of psychological states, while the physician constantly ascertains the state of function by inquiring into the feelings of the patient. As, moreover, he next verifies this by a physical and chemical diagnosis, we become convinced of a complete constancy or parallelism of normal and pathological relations between chemico-physical changes, organic functions, and psychic states. The materialistic interpretation—i.e. of the latter two in terms of the first—is, however, as Comte has especially insisted, as unverifiable as would be the converse transcendental one. The links of the argument that physical changes set up the mental ones, may with equal strength be placed in precisely reverse order, to prove that psychic changes set up the physical ones. Given in fact these three orders of phenomena, physico-chemical, biological, and psychic, the labours of physiologists have done great service in establishing detailed parallelism between all these processes, yet leave the original distinctness of each category unimpaired. We can not only correlate the digestive process with normal psychic states of hunger and satisfaction, but define its physico-chemical constants, and even experimentally reproduce these in our test-tube outside the organism altogether. In a definite and very important sense, therefore, we can now say that we understand digestion in terms of chemistry and physics; but this advance of our physiological chemistry is apt to produce serious misunderstanding when it prevents the chemist from seeing that his biological colleague is not ultimately concerned with the action of the acids and ferments, in terms of which the chemical 'explanation' of digestion is expressed, but with digestion as one factor of his biological unity, the organism—as one process of its 'life'—i.e. of its adjustment of internal and external relations, a complex process with which no test-tube can present the slightest parity. Similarly, the psychic state accompanying digestion, whatever its precise physico-chemical and functional concomitants (although it is of course most interesting and desirable to define these), is only truly known by reference to the whole succession of phases or forms of mentality, which it is the problem of psychology to analyse and classify. It similarly, therefore, eludes all attempt at expression in terms of the contents of our test-tube, unless, indeed, we go so far as to endow this or its contents with specific mentality as well: this, however, would of course be an unverifiable hypothesis of transcendentalism, not a scientific materialism at all. In short, then, the physiologist is entitled to say that he knows of 'no thought without phosphorus,' but by no means that 'the brain secretes thought as the liver does bile.'

Again, since in sociology our problem is of social aggregates, the study of the individual may and does indeed furnish the most valuable data, but our ultimate sociology—i.e. our generalised and unified science—is concerned not with the individuals but with the essential drama of the race. The final reductio ad absurdum of the crudely materialistic position is afforded by pushing it to its logical consequence, for if, as we see many disposed to think, social phenomena fall truly within the province of the biologist, and this be but the interpretation of molecular machinery in terms of chemistry and physics, then the ultimate generalisations of sociology, the philosophy of history, would have to await elucidation by the organic chemist! It will thus be sufficiently clear that the parallelism which we are constantly establishing between all orders of phenomena, physical and biological, biological and psychological, or these and social, and which we may define as the province of a legitimate materialism, in nowise interferes with that claim to the unity and distinctness of each of these sciences, which remains as the stronghold of an equally legitimate transcendentalism.

6. Generalisations of Biology.—But while all these provinces of the science must be separately discussed, their results demand unification. It must not be forgotten that science, no less than music, is to be distinguished from its printed records, and even from our power of recollecting its details piecemeal. For the measure of our grasp of biology lies in our power of simultaneous orderly presentment of its whole body of essential truths before our minds, and such a breadth of synthetic vision, although demanding an exceptional effort, is becoming at least partially possible.

If we thus attempt to reach the largest possible general view of the science, we have evidently not only to unite the results of the morphological and physiological study of the organisms before us at the present moment, but to introduce the conception of past, indeed also of future time as well. This entire drama of organic existence is what we have now learned to call its evolution; and consequently, since this supreme generalisation has been clearly laid hold of by the students of separate sub-sciences, these have been well-nigh transformed. Their unification and detailed progress have alike made enormous advances, and we are thus rapidly approaching a unified and systematic account of organic nature.

Within the present limits only the merest outline of this synthetic view can be attempted. Starting once more at the rise of modern botany and zoology, we see the naturalists of the renaissance slowly disencumbering themselves of the literature of mere medieval traditions distorted from the classic past, which did duty for natural history, and returning to nature. We watch the rise of the systematic movement from Ray to Linnaeus, and thence trace its extending exhaustiveness, aided by new appliances like the deep-sea dredge and the microscope, new facilities like the marine station and the exploring expedition, and carried on by an ever-increasing multitude of workers. Accepting this catalogue of organisms, which, though still incomplete, is now for most purposes far more than adequate, we have next to sum up each of the successive planes of morphological and physiological research, and seize the results of the scrutiny and comparison of the form and function of organs, tissues, cells, and protoplasm of the multitude of organisms which have now been submitted to this detailed analysis. An attempt at the synthetic presentation of this must set out from the Protoplasm (q.v.), which is the 'common denominator' or 'physical basis' of life. Here we have to acquaint ourselves with the conditions of its existence, and take note of the physiologist's enumeration of its functions as 'contractile, irritable and automatic, receptive and assimilative, metabolic and secretory, respiratory and finally reproductive' (see PROTOPLASM).

The study of the unit-mass of protoplasm or cell should next be summarised, its varied forms surveyed and classified from the simplest to the most complex and many-celled plants and animals, and these generalised into the conception of a simple 'cell-cycle,' ranging between a more or less passive and encysted spheroidal form to a less or more motile (amoeboid or elongated) one. The phases of cell form are also seen to arise as so many stages of the rhythm between growth and waste (anabolism and katabolism) into which, as we have seen above, all functions are finally summed (see CELL). The simplest organisms are separate unit-masses of protoplasm, of which the life-history essentially coincides with the cell-cycle aforesaid (see PROTOZOA). In higher but still unicellular forms, the resting or the motile phase becomes more and more permanent and specialised, and a distinctly plant or animal character thus makes its appearance. Multiplication soon becomes habitual at some one definite phase, and instead of the separation of the resultant portions to lead an independent life, we at length have their continued union. Such a union of normally resting cells gives us the multicellular plants or Metaphytes; that of normally amoeboid cells, the multicellular animals or Metazoa (q.v.). By the more passive vegetable organism the energy of the sun's rays is usually absorbed by aid of a green colouring matter (Chlorophyll, q.v.), and applied to the reduction of the carbonic acid of the atmosphere, starch and cellulose being also normal products, while their nitrogenous matter is obtained in solution as ammonia or nitrates. On the other hand, the more active animal is dependent for its nitrogenous food on proteins already directly or indirectly worked up by the agency of plants, and also for its carbonaceous on organic substances, starch, sugar, fat, &c.

Of the multicellular plant or animal only a few cells remain comparatively undifferentiated to form the reproductive elements. In these that alternate or permanent preponderance of katabolism or anabolism, which is a constitutional feature of the adult life of the great majority of living beings, usually specially asserts itself; and we have thus the development of those motile and resting forms which are henceforth respectively characteristic of the male and female 'sex' (ovum and spermatozoon). See REPRODUCTION, SEX. After a more or less distinct 'rejuvenescence' of the primeval cell-cycle, usually accompanied by that complete union with a cell of contrasted sex which we term fertilisation, the resultant cell (fertilised ovum) undergoes segmentation into an embryonic mass (see EMBRYOLOGY). From this a new adult organism gradually develops. In the animal this embryonic sphere becomes dimpled into a two-layered sac, the primeval stomach or 'gastrula'; a third intermediate layer arises, and from these three certain definite organs constantly develop. The outer furnishes the epidermis, nervous system, and sense organs; the inner layer the alimentary canal and its appended glands; and the middle one the skeletal, muscular, excretory, and reproductive systems. This unit of origin gives us a starting-point for identification of like organs in different animals, and the fertile study of homology thus begins. Again, this development of at first similar cell-unions into tissues and organs is readily explained as a consequence of the division of labour in the individual and in the race. The associated units have their activities restricted in one way or in another, and their energies find new outlets varying with their special circumstances. Function establishes structure, and these gradually perfect each other; the tissue is thus formed, and by-and-by becomes rounded off or combined with other tissues into a definite mechanism, the organ, while these may form a still larger unit, the system. It is therefore the development and complexity of this physiological progress of division of labour which determines the special and general form of the organism. We thus see how the results of pure morphology (anatomy without regard to physiology) acquire a rational explanation by reference to the physiological processes which shape organic forms. With increasing complexity of structure we have necessarily an increasing simplicity of function, for the many processes at first crowded into a single cell become, so to speak, sorted out into the different organs. Residual traces of all these protoplasmic properties must, however, remain in varying measure in even the most specialised of living cells, hence the possibility of functional change is never closed, since use, disuse, or changing circumstances may set up a renewed increment of any one or a continued decrement of any other of these minor processes, however apparently suppressed. Here in this latent potency of change we have in short the rationale not only of the cell-cycle with which we set out, but the explanation of the otherwise mysterious phenomena of reversion and progression—in fact, of the occurrence of all variations, normal or pathological alike. See PHYSIOLOGY, PATHOLOGY, VARIATION.

It is clear that this specialisation of organs involves their dependence upon each other, and that if one suffer or develop, this must affect all its partners in the general life. Hence we obtain 'correlated variations,' and interpret the various symptoms of a given disease. Progress as regards organs is thus twofold—(1) in continued development and increased division of labour of the parts (differentiation); (2) in increasing correlation of the parts to each other and in subordination to the whole (integration). And when we note that any higher integration depends upon a preceding new differentiation, which of course disturbs the existing equilibrium of the whole, we are in a position at once to comprehend the beginnings of disease, and to realise that the traditional pessimistic interpretation of this admits of important modification.

A strict classification of the organs is rendered difficult by the varied ways in which different organisms have solved the problems of adaptation: a general correspondence to those essential properties of protoplasm which we noted at the outset is of course observable, since the frog has no functions which were not foreshadowed in the amoeba. The older physiologists, however, clearly pointed out that the primary classification is that which separates the reproductive or species-regarding functions from the remaining individual ones; while the latter are again divisible according as they are concerned with the internal problems of the organism—i.e. with nutrition (also circulation, mechanical support, &c.) or with the external problems of relation to the environment (sensory, nervous, muscular). These fundamental contrasts between reproduction and individual development, and between the vegetative and the animal functions, are in fact the largest physiological features of organic nature, and admit of fruitful application (see VARIATION) to interpret the endless details of plant and animal form and life.

Returning to these, which we are now in a position to view as an ordered whole, we find that (a) the successive appearance of related types upon the globe broadly (and sometimes even minutely) corresponds to (b) the order of increasing complexity established on purely anatomical grounds, while the embryologist (c) finds that the development of the individual from its unicellular simplicity to adult complexity repeats the same essential succession of phases. This threefold coincidence next demands explanation, and the 'theory of descent' from a flora and fauna of increasingly simpler character in the past necessarily makes its appearance; nor has any other scientific explanation been suggested. But this modification during descent next demands explanation; a problem of supreme difficulty which has had many attempted answers.

From all these repeated attempts to discern the principle of organic progress, the naturalist has been learning to change the static or anatomical view of living nature for the dynamic and physiological one—in other words, to rise from the analysis of the individual as a dead specimen to the conception of the species as a living whole. The commanding superiority and wide scientific influence of Darwin among naturalists are of course popularly, though groundlessly, associated with the origin instead of the final popularisation of the conception of descent; while even by scientific men they are frequently assumed to be essentially associated with his particular hypothesis of natural selection as the rationale of progressive modification; yet his biological influence and example primarily depend on his having fairly established in practice this great transition from the pursuit of dispersive specialisms towards attempting a general view and explanation of organic nature. Since his time, our knowledge of the plant can no longer be grouped round the dried herbarium specimen of it, as had been the case since Linnaeus' day. It has become again for us a living being, not only breathing, and growing in the sun, but moving, digesting, sensitive, often in a measure hardly second in its fullness to that of animal life—in a word, the dryad of old poetry has been recalled to life by modern science. The individual, too, has wide relations, here it may be to other plants, here to insects, there to birds and beasts: the entomologist must leave his museum, and the botanist his herbarium, to work out in the garden together the steps by which flower and plant have wrought out each other's form and destiny: they have soon to call in other specialists to their aid; and the world thus comes to be understood as a vast and complex web of life, and its many forms as related to each other by innumerable complex ties. Thus the farmer's cat influences his clover crop, or the distribution of a particular insect determines alike the civilisation and the scenery of vast regions of Central Africa. In this way the unit-problem of the science has become no longer the description of the isolated specimens of its index collections, but the history of the entire species in its living relations, actual and historic.

Since the doctrine of natural selection, which is the more immediate contribution of Darwin and his school to the theory of evolution, will be found summarised under DARWINIAN THEORY, and discussed under EVOLUTION, it suffices here to point out that, despite its wide applicability and indisputable services, its finality is again becoming disputed by various naturalists on different grounds (see EVOLUTION, VARIATION). On one side the older explanations, in terms of use and disuse, environment, &c., are being revived; while on another, it is being attempted to replace the received doctrine of indefinite variation, with progress by means of struggle for existence among individuals, by the conception of definite variation (even pathological), with progress essentially in the measure of the subordination of individual struggle and development to species-maintaining ends, reproductive, domestic, or associative. Without entering into details, it is evident that such a restatement of the theory of the evolution of living beings—in terms no longer primarily of strength and competition, of hunger and battle, but of love and co-operation, of sacrifice and pain—would involve, no less fully than has the doctrine of struggle for existence, a deepened reinterpretation of plant and animal life, and would similarly extend into other fields than those of pure biology.

7. Conclusion—Biology and Practical Life.—While biology as a science finds its source and justification in the natural impulse to contemplate, and reasonable desire to comprehend the world and our place in it, its action and reaction with practical life requires brief consideration. Our knowledge of animals, like the child's, obviously arises with their chase; and that of the aspects and properties of plants, wholesome and poisonous, perhaps even medicinal, with the hungry search for roots and berries. The evolution through higher social states finds its reflection in widening zoological and botanical folklore, and the developed agricultural conditions of civilised life not only admit of the increasing and systematising of our knowledge, but even at length contribute valuable conceptions, like that of selective breeding, of which Darwin has made such especial use. The recent contributions of biology to the arts of life have been of course primarily associated with the advance of medical treatment; hence the popular and even medical conception of the botanist is still based upon the traditional one of the herboriser in quest of specific remedies. The increase of food-supply, through pisciculture and breeding, and through the destruction of the enemies of useful species, is an application of more recent but widening growth; in fact those applications of our knowledge of cryptogamic pests which have especially culminated in the labours of Lister and Pasteur (see GERM THEORY), at present furnish the stock illustration of the applicabilities of pure biology. New ideas are also germinating; thus speculation is busy—e.g. with schemes of artificial human selection; while rapid progress is being made in the transition from detailed medicine to wholesale hygiene—i.e. beyond the mere application of specific remedies to morbid individual variations, and towards a progressive and harmonious reorganisation of the functions and environments which are afforded by the human hive or city to its individuals. For the task of biology applied as hygiene should not only be to subdue the tendencies to morbid variations, but to determine that maximum of optimum life-cycles, which is the biologist's and hygienist's concrete rendering of the greatest happiness of the greatest number. And as the twin scientific motives of childlike curiosity, with its analytic pursuits, and of maturer contemplation, with its synthetic aims, become balanced by the corresponding practical ones of the arts and their application to the general humanist problem just outlined, our biological science and art may be fairly expected to react more and more fully in their associated yet alternate progress.

This hope is indeed confirmed by the whole past history of the science; with a glance at which in its most general aspect the present article may fitly conclude. At the very outset (§§ 1, 3) we saw that biology is no mere isolated stream of fortuitous discovery; next (§ 5), that it is no mere separate discipline which can be exhaustively pursued apart from other sciences; and—now (§ 7), that it has arisen from, and constantly returns to, ordinary life and practice. In tracing the progress of biology, we are simply following the reflection of the changing lights cast upon the organic world by each prevailing mode of general thought and social life. In a word, the evolution of biology forms part of the general social evolution; the science is no completed body of truth, but merely such portion of it as our stage of social progress enables us to see. Else the rise of science from art would be little more than an almost prehistoric process, instead of being still and continually going on. Innumerable instances, large and small, might be given of this; thus, the classificatory doctrine of the 'échelle des êtres' (see ZOOLOGY) due to the naturalist Bonnet, is far more than a mere detail of the biographical history of zoology; for the conception of an unbroken series of beings ascending in regular gradation from the lowest up to the highest is obviously the projection upon nature of that established ecclesiastical and social hierarchy in which the good abbé's mind was formed. Again, taking a larger instance, the substitution of Darwin for Paley as the chief interpreter of the order of nature is currently regarded as the displacement of an anthropomorphic view by a purely scientific one: a little reflection, however, will show that what has actually happened has been merely the replacement of the anthropomorphism of the 18th century by that of the 19th. For the place vacated by Paley's theological and metaphysical explanation has simply been occupied by that suggested to Darwin and Wallace by Malthus in terms of the prevalent severity of industrial competition, and those phenomena of struggle for existence which the light of contemporary economic theory has enabled us to discern, have thus come to be temporarily exalted into a complete explanation of organic progress.

Finally, the division of labour having become fully established in industrial practice, and recognised in economic theory by Adam Smith, it was frankly borrowed for biological application by Milne-Edwards, almost a couple of generations later, with fruitful results. This industrial development has indeed not only given us our present clear conception of separate organic functions, where an earlier school could see only their general resultant as 'temperament,' but it has also determined the prevalent intensity of scientific specialism within artificially restricted fields. Hence too, the extreme specialist's not infrequent loss, if not indeed denial, of definite responsibility to the science as a whole, and still more to that larger progress of which it forms a part, is simply the equivalent of that loss of conscious relation both to the special task and to its general bearings, from which at present the labourer also so frequently suffers. But as we progress with the synthetic reorganisation and practical application of the science, we may anticipate the recovery of both.

The manifold importance of biology in education is seen not only in its practical applications in the arts and in the study of medicine, but as a potent agency of culture, and as preliminary to psychological and social studies. See BOTANY.

Littérature.—See, beside articles mentioned above, LAMARCK, SPENCER, WEISMANN, &c.; also Comte, Philosophie Politique; Robin, La Philos. Pos.; Laffitte, Revue Occidentale, i.; Spencer, Principles of Biology (1866); Haeckel, Generelle Morphologie (1866); Huxley, Anatomy of Invertebrate Animals (1878); text-books by J. R. A. Davis (1888), R. J. H. Gibson (1889), T. J. Parker (1891), H. G. Wells (1893); J. Arthur Thomson, The Science of Life: an Outline of the History of Biology (1899); Alfred Earl, The Living Organism (1899).