Popular Science Monthly/Volume 8/March 1876/Modern Philosophical Biology I
By Dr. E. GAZELLE.
TRANSLATED FROM THE FRENCH BY J. FITZGERALD, A. M.
BIOLOGY, or the science of life, is so new a subject of investigation that its limits are as yet imperfectly ascertained. Metaphysical ideas have too large a place in our conception of its extent. When we ask where biology commences, we are met by the problem of the origin of living: things. which very often is solved in accordance rather with preconceived opinions of the system of the universe than with an independent scientific hypothesis. When we would determine its limits, we are met by the problems of cognition, and of the causes determining man's actions; and again usually it is unscientific prejudices that decide whether these problems should be referred to another science, or treated under a subdivision of biology; whether we should range, alongside with phenomena which unquestionably belong to biology, those other phenomena which experience shows us to be closely connected with them, associated with them, and which are in such constant ratio with them in their variations that they appear to derive from them, and from no other source, the conditions of their existence. The indecision as to the limits of biology results principally from the difficulty of giving a strict definition of its subject-matter. Still, in spite of these difficulties, though we cannot say precisely what life is, where its province commences, where it ends, there exists between the two extremes—the inorganic world and the mental world—a very firm ground, imperfectly explored, it is true, but nevertheless belonging to biology alone. The various departments which constitute this domain, though they themselves are not all very clearly defined, are sufficient to give to biology a definite individuality.
Living things present themselves to the observer of Nature as individuals; and it was not long before man began to regard them from another point of view, as forming groups of similar individuals more nearly allied to one another than to individuals in other groups. At first these groups were held to be natural; next it was asked whether, like individuals, they had a history—a beginning and an ending. This question, as being one that strikes the imagination, naturally arose even before science possessed the means of settling it, and preceded, in the historical order, that thorough study of individuals on which its solution really depends. When men of science had begun to study living things with other purposes than simply that of deriving from them knowledge that would be available for the medical art, and had gained sufficient information for inductive generalizations, they no longer contented themselves with theories of the origin of groups, but sought to reduce to general principles the structure of living bodies—a thing which previously had been considered only from the topographical point of view, and with reference to what was called the use of the parts; and on these general principles they sought to rest a scientific theory of the origin of natural groups.
A man of keen and powerful intellect, who, had he but lived in our time, would have attained the summit of fame, with marvelous acumen anticipated a doctrine which is steadily tending to become a received scientific theory, viz., that the changes which have occurred in Nature are the effects of constant natural laws. Applying this idea to the natural groups of the animal kingdom, he rejected the hypothesis which ascribed to geological catastrophes the destruction of entire faunæ, and the preparation of the earth's surface for a fresh special creation. The transformation of lower organisms into higher he referred to the action of modifications which, though in themselves inconsiderable, became important from repetition and long accumulation, under the influence of forces whose powers he exaggerated. Species and varieties he regarded as artificial groups. According to him the very simplest organisms are derived, by way of spontaneous generation, from naturally-produced plastic substances; then they mutually diverged by imperceptible differences, so as to constitute a linear series, which, but for the gaps caused here and there by lost species, would present to us the aspect of a continuous system. Under favoring circumstances the organs of an animal are modified; a change in the circumstances causes changes in the structure of the individuals belonging to a species, and is the starting-point for the formation of a new species. Crossing, by producing hybrids, still further multiplies the number of species. And species appear to be fixed, simply because the circumstances appear to be similarly fixed during the brief period embraced in our observations. Transformation is the rule, and in the regular course which it runs we can discover no indications of plan or purpose.
The ideas of Lamarck, being but ill supported by positive proofs, were looked on as mere speculations, plausible but doubtful, or even as dreams, unworthy of science; his generalizations were discredited, and even now, when they reappear, backed by a powerful array of facts, but few ever think of giving due honor to their author.
The attempts made at the same period to form generalizations with respect to the constituent arts of the living individual were more successful. It was not enough to know in a general way that the phenomena observed in living things are in the last resort the same in kind as those which are known as physico-chemical, and that they obey the same laws. Between the phenomena of living things and those of inanimate Nature there existed too wide a chasm; there was no way of passing, deductively, from physico-chemical laws to vital phenomena, and the scientific explanation of organic forms and of functions was of necessity defective. The author of the "Anatomie générale" simply recognized in organs various elements, which he grouped in families, with a view to define, under the general name of tissues, the basis of their structure. In these elements he recognized, independently of their physical and chemical properties, special properties which he justly denominated vital, inasmuch as it is by them that life manifests itself, and which are, properly speaking, the function of these elements. Bichat's generalizations were, doubtless, in his own mind, in opposition to the theory which refers vital phenomena to physico-chemical properties; in point of fact, they have established a relation between functional facts and the general properties of matter. The functional facts of organs are explained by the elementary properties of the tissues; and the latter, though we cannot as yet refer them to physico-chemical properties, are, nevertheless, brought into remarkably close relation with them through our modern ideas of the constitution of organic substances and the principle of the equivalence and transformation of forces.
Still, these relations could not be perceived prior to the discovery of the relations which connect organisms and their tissues with external forces possessed only of physico-chemical properties; and this conception dates from a time long after Bichat's day. We have reason for believing that the part assigned by Lamarck to the action of external circumstances upon organisms first suggested this conception, owing to one of those mysterious operations of the mind which, out of an idea vaguely descried, and even, perhaps, not accepted in the form in which it first presented itself, forms a nucleus around which experience and reasoning group proofs, and which the inventive faculty develops under the form of a doctrine apparently brand-new. The doctrine of the action of "general external modifiers," which Blainville sets forth summarily in his "Cours de Physiologie générale et comparée," by no means possessed, even in his own mind, all the importance it later assumed in science under the name of "doctrine of media," after Auguste Comte had given it so prominent a place in his "Biologie." But, by bringing upon the scene the action of external circumstances upon the sum total of a living organism, and by calling attention to the effects they produce therein, whether as stimulating or reviving the functions, or as suspending the same, Blainville prepared the way for a better interpretation of vital phenomena; and though he himself, with all this light, did not attain to the truest conception of life, he nevertheless broke ground for those who afterward were to do so.
In more recent times biology has been enriched with an enormous amount of facts for which we are indebted to the labors of naturalists, or even of mere breeders, as also to the labors of anatomists and clinicians, but, above all, to researches in experimental physiology, wherein the application of physical-science methods to the discovery of the laws of vital phenomena has been attended with brilliant success. Amid the extreme complexity of these phenomena it was difficult to perceive the relations of succession which unite them, and to establish positive series. But when men of science refused any longer to content themselves with observing them as they occur spontaneously, and began to vary them by calling in the action of special agents, then modifications were produced, the true causes of which were easily recognized. As in the study of inorganic bodies we learned the laws of their actions and combinations by seeking to find out with the aid of reagents—which are, in fact, special modifiers—the way in which they behave under circumstances that are well known, being fixed beforehand by the observer; so, in the study of living bodies, the introduction of experimentation which alters, according to a plan determined beforehand, the conditions under which the functions of life are to be performed, has enabled us to perceive, with an exactitude previously unknown, the organic properties underlying these functions. Even in embryogeny, a science which once seemed to belong to the domain of simple observation, it has been possible, by way of experimentation, to gain results which shed some light upon teratology. The employment, in observation, of instruments of precision, and in particular of registering apparatus, and of all those processes which suppress causes of error resulting from the personal peculiarities of the observer, gives to the results of research a degree of certitude which renders indisputable facts properly so called, the only question that remains being as to whether these results have been rightly or wrongly interpreted. In addition to an immense amount of unquestionable facts, in addition to a knowledge of the elementary properties of organic tissues and an acquaintance with the special laws which represent the action of these tissues in presence of these modifiers, this general result has followed the conquests of biology, namely, that living bodies are now known to be subject to the self-same laws which govern inorganic bodies, and that, under the hand of the experimenter, the course of things within the tissues is precisely the same as without the tissues; that in the laboratory the elements of living bodies, like those of inanimate things, have their own way of affecting the mind that observes them—that is to say, they possess fixed essential properties which can be determined; and what remains yet to be known is, above all, the mode in which those organic substances are formed which are the basis of living: bodies. The belief which from day to day is gaining confirmation from the labors of physiologists is that so boldly expressed by Claude Bernard, viz., that as the chemist, starting with the knowledge of inorganic bodies, subjects them to his will and creates new bodies, so the physiologist, starting from organic matter, "by imposing upon it special conditions, will be able to produce new physiological modifications and new series of phenomena, thus modifying at will living-bodies, and even creating them."
At the same time, by comparing and analyzing the different branches of biology, certain very general laws have been established, particularly in physiology proper, having a bearing upon the development of the individual and the relations of the functions to their organs. We are in possession of a certain number of very broad though purely empiric generalizations on the phenomena upon which the superiority of living things over one another depends. These are, properly speaking, laws of organic Nature.
First, we have the law of the increase of the mass of the organism, in virtue of which each living thing attains its full development only by passing through a series of phases characterized by an augmentation of its mass, and consequently by an augmentation of the quantity of force applicable for its physiological actions, as also by an augmentation of the quantity of functional products.
Then there is the law of the multiplication of parts in proportion as we ascend in the series of living things, this multiplication being determined by an increase of complexity in the organic machine, in virtue of the diversity both of the functions which make their appearance and of the organs which result from this diversity of functions.
Again, we have the law of coördination and subordination of functions and organs, in virtue of which, in proportion as complexity is introduced into the organism and as the functions and organs take on a more special character, certain functions and the organs performing them become dependent on other functions and other organs. Besides, a tie of solidarity is established between all the parts of the living body, so as to guide them toward a common end, the conservation of the individual, while at the same time all of the parts feel the reverberation of the actions to which each is subject.
Next comes the law of adaptation, in virtue of which an organism tends to be so modified as to seem to be specially created to suit the circumstances amid which it exists and the kind of life imposed upon it by them. This law is still, for many thinkers, the basis of ideas of final causes by means of which they strive to explain the structure of living things and the variations observed therein.
Finally, there is the law of heredity, in virtue of which organisms produce new organisms which repeat their type. Heredity is the law of fixity; it expresses the tendency to perpetuate a condition of things which is itself the result of past environments, and to set it up as a barrier against the influence of new environments.
Descending still deeper, scientific men have sought to explain the constitution of living things, their production, and the existence of the groups into which we find them divided. Hence three theories which have had different fortunes—the cellular theory, the doctrine of spontaneous generation, and transformism.
Schwann, applying to the animal organism Schleiden's discoveries in vegetal organisms, showed that the tissues are formed of primordial, i. e., irreducible, elements, called cells, though often these elements have no cavity and are simply rounded masses. The egg, which is the starting-point of all animal organisms, is at first merely a cell, and develops by producing within itself other cells, which are the primitive materials of the living being. All that the organism is comes ultimately from the cells, which are converted into living tissues. They adhere to one another end to end, and become flattened, or lengthened, or ramified; or they unite and form one common cavity, keeping their walls only at points where they are not in contact, thus forming tubes, or fibres, as, for example, in the histological elements of muscles and nerves.
Some authors have explained the production of cells on the hypothesis of a true spontaneous generation. According to them, cells are organized in a saline solution, the first step being the deposit of a nucleolus, around which there forms an envelope called the nucleus, and finally, at a greater distance, a second envelope, or cell-wall. But no actual experiment has ever been made on the production of cells in this way, and hitherto we have no knowledge of a cell being produced save from a cell. Of this famous theory so much yet remains, viz., that the cell, whatsoever its form and whatever modifications it may have received, is ever the basis of the vital phenomena.
"One only elementary form" (says Virchow) "runs through the whole organic world, remaining ever the same; in vain would we attempt to substitute any thing else for it; there is nothing that can take its place. We have come to regard even the highest formations, whether plant or animal, as being the sum of a larger or smaller number of like or unlike cells. The tree represents a mass put together according to a certain law; each of its parts, leaf or root, trunk or flower, contains cellular elements. The same is true of the animal world. Each animal represents a sum of vital units, every one of which has in itself the perfect characters of life. . . . The higher organism, the individual, is always the result of a sort of social organization, of the union of sundry elements combined; it is a mass of individual existences, dependent on each other, though their dependence is such that each element has its own proper activity; so that, whatever impulse or excitation other parts may give to the element, the resulting function nevertheless emanates from the element itself, and is its own."
The question as to how living bodies arc produced gave rise, a few years ago, to discussions which have again brought to the surface a doctrine which was supposed to have been disproved two hundred years since, and which reappeared in the last century only to be assailed with Voltaire's sarcasms. I mean the theory of spontaneous generation, so called—a self-contradictory phrase, by which it was intended to assert that organisms are produced out-and-out without the aid of parents resembling them. While admitting that generation, sexual or asexual, is the mode of reproduction found among animals possessed of complex structure, the partisans of spontaneous generation held, on the strength of their experiments, that certain very low organisms might be developed spontaneously, without specific germs, in infusions of organic substances. But though in this dispute experiment has given no definitive verdict—nor, indeed, was such verdict to be expected—still, all the probabilities are on the side of those who assert the universality of generation by means of germs developed in the parents; and, in the absence of experimental demonstration, we are not without theoretic arguments against the spontaneous generation of the comparatively high organisms developed in infusions. If this doctrine is to be retained, it is not for the purpose of explaining the formation of organisms, a thing well enough explained without it, but in order to account for the production of really primitive living things—i. e., for the appearance of life in a fraction of organic substance, whether this is still possible in our day, or whether it was possible only at a time when, under conditions unknown to us, organic substance originated upon the earth. Thus stated, the question does not depend on experimentation; it becomes a mere exercise of the imagination, and the result is valueless.
Whatever is to be thought of the theory of the beginnings of life, one or more first living beings having appeared upon the earth, after the latter had become capable of supporting them, the question arises as to the transition from the primitive simplicity to the enormous degree of variety now existing. Here we have the problem of the origin of species, which is solved by the theory of descent, sometimes denominated transformism. The old conception of living Nature as an infinitely varied assemblage of organisms which faithfully copy certain types, all of whose parts are governed by the law of final causes, in our time gives way, not without a fierce struggle, before a new conception, which represents living Nature as an infinitely varied assemblage of organisms which are ever varying under the influence of external circumstances, while under the influence of heredity they tend to fix in a type the results of previous variations. At one time we have, as in breeding, artificial selection; at another time, as among people who have not yet discovered the laws of breeding, a selection that, though unsystematic, is nonetheless real; finally in Nature, without human intervention, a selection based simply on the conditions of existence. In natural selection, the action of which is by far the most general and powerful, the fixing of variations results from adaptation to the existing conditions. This adaptation finds expression in the survival of the fittest in the struggle for life; that is to say, those individuals continue to live and reproduce their kind whose structure enables them to undergo changed conditions without succumbing, while others, because they cannot adapt themselves, perish, leaving no posterity, no trace of their having ever existed, save, perhaps, in the geological strata of their epoch. The special advantage which has once insured the survival of an organism, while its congeners which possessed no such advantage perished, is fixed by heredity; it grows under the influence of that same law of survival which insures the upper-hand in the struggle for life to the organisms possessing the advantage in the highest degree; in virtue of the law of the coordination and subordination of parts and functions, it brings about in the whole organism very extensive modifications which insure its fixity; and the sum total of the new characters becomes sufficiently stable to convey to the mind which observes it the impression of the persistence of forms and the existence of types, whereas in fact there exist only changes amid which there remain, in virtue of the law of heredity, traits of resemblance to a common ancestor or stock.
Such are, in brief, the principal laws of biological phenomena, and the chief theories which have been devised for the purpose of assigning to them causes. When, in order to establish or to impugn laws and theories so far-reaching as these, we can have recourse to direct experiment and observation, the mind is satisfied and its certitude reposes on an immovable basis. But when a theory has to do with origins in the remote past, or even in the present, but inaccessible to experiment, our certitude rests on no solid foundation. In the absence of experiment, we have to be content with opinions formed according to the rules of induction and of analogy, and possessing more or less probability. Among views of this sort, those appear to have greatest weight which, in their contexture and in the method of their formation, are most in harmony with those beliefs of which we are most certain; which rest on the same general principles; which, so to speak, are incorporated with our beliefs, so that, were they to succumb to criticism, their fall would compromise the entire system. In other words, they must occupy their own place in a general philosophy, there appearing as so many links in a chain attached, on the one hand, to laws and theories which account for them, and, on the other, to laws and theories which without them cannot be explained.
Could we look for this result from the only general system of philosophy which has existed down to the present day? Having been written at a time when the science of life had for its generalizations only conclusions from Bichat's researches, the hypotheses of Gall, and the results of classification, that portion of the positive philosophy which treats of biology is too far behind the actual state of science to serve as its guide; yet, owing to the largeness of the views there expressed, Auguste Comte gave to this work a comprehensiveness which enabled it to take in some of the great biological systems elaborated in recent times, and one of his followers has recently declared that the success of these doctrines does not impair the unity of the positive philosophy. It can also be truly said that, if those doctrines were to succumb, the positive philosophy would suffer no loss; and this proves that they have no connection with this philosophy, and that they can receive no support from it. Still, in spite of this serious shortcoming of his philosophy, the services rendered by Auguste Comte are very great. He has given a better definition of life than the one then in vogue; he has perceived that life is a continuous chain of chemical facts, and to this doctrine he has given forcible expression; lie has illustrated, by judicious contrast, the relations of the organism to the medium in which it lives; he has stated with great precision the problem of the science of life, which consists in expressing in the least number of laws of the utmost generality the harmony which unites the organism to its medium by vital acts; he has forcibly shown the close correlation which enables us to infer the function from the organ, and vice versa; not to speak of a multitude of useful and profound considerations upon the structure of living bodies, on comparative anatomy, and on the physiology of the functions of relation. But it was characteristic of Auguste Comte's philosophy to bind together the parts of its system only by a purely logical tie, and not at all by establishing relations between the phenomena, or by showing interdependency of laws. For him it was enough, in order to assure to biology its place between physico-chemistry and sociology, if on the one hand a knowledge of physical and chemical laws is necessary for the study of biological phenomena, and if the various classes of phenomena pertaining to these sciences really act a part in the production of vital phenomena; and if, on the other hand, a knowledge of the life of relation in its highest aspects, i. e., in the cerebral apparatus, and the elementary intellectual and passional faculties corresponding thereto, is an essential preliminary of the study of sociology. Hence, the biological work of Auguste Comte has not per se had any great influence on researches of this kind. The general current of his philosophy has exerted a good influence in so far as it has disinclined men toward theological and metaphysical explications. But we cannot admit that Comte has founded a philosophy of biology fitted to inspire or to guide research. Biological research is still what it was before the positive philosophy became popular; it is still restricted to special points; and, though its spirit is becoming more and more positive, the reason is because in such research the imagination is brought more and more under subjection to the laws of scientific investigation. But, meanwhile, we see no indications of philosophic purpose, no aiming to bring the results obtained under the dominion of a more comprehensive law.
It appears to us that, if a philosophy is to assume this rôle and to undertake the guidance of man's thought and action, it must bring forward general principles of such breadth that they will apply to all orders of phenomena, from the simplest to the most complex—a system of laws coördinated by deductive relations, and by its universality expressing all the phenomena of the universe. Whether these general principles are given a priori, as the intuitionists hold, or whether they are the abstract expression of an experience invariably and unconditionally repeated, at all events they must be such that from them all our scientific theories may be deduced; they must appear in all our researches as the criterion of the truth of the results, and they must underlie all our anticipations of truth as the guiding principles. Causes, that is to say, the sum of the antecedent phenomena, whose joint action is necessary for the production of the consequent phenomenon, or effect, may be as diverse as you please, nevertheless their relation to their effect will be expressed by the same general law.
A philosophy of biology must reduce under these principles of philosophy all the truths furnished by experience in the various branches of investigation pertaining to that science; must explain them by these principles; must present them to us as necessary, and the contrary results as illogical and unphilosophical, so as to produce a twofold effect, viz., the highest possible harmony in the system of our knowledges, and an ever-strengthening confirmation of the general principles which are their abstract expression. We must demand of it a verdict upon doctrines respecting the constitution of the living individual and its origin and the constitution of the species to which the individual belongs, which verdict shall oblige us to accept these doctrines as corollaries of the same general principles from which the accepted theories of the other abstract sciences are likewise deduced. Finally, we must derive from this philosophy of biology the assurance that the generalizations which it offers to us are grounds upon which we can stand securely in our deductions—of course within the province of biology—respecting man and the human species.
Mr. Herbert Spencer attempts something like this when he rests the laws of biology upon the theory of changes in the course of things, as set forth in his "First Principles." The "Principles of Biology" is the first application of his system of philosophy to a highly-complex order of phenomena.
It will be well to give a sketch of Mr. Spencer's whole system, so that we may better understand the meaning of the abstract terms he employs, and the relations between the general laws on which the system is based. We shall thus be in a position to appreciate the author's application of his system to the more restricted field of biology.
Underlying Spencer's system we find the principle of the persistence of force, "the sole truth which transcends experience," to which "an ultimate analysis brings us down," and on which "a rational synthesis must build up." From this first principle come as consequences two correlative principles, viz.: uniformity of law, which is simply the persistence of the relations between forces, manifested under identical forms and conditions; and the principle of the equivalence of forces, inductively established within the last twenty years. The researches which resulted in the establishment of this principle rest implicitly on the persistence of force, inasmuch as they measure all the precedent forces, which have disappeared, and all the consequent forces, which have been produced, by the aid of a unit supposed to be constant. If we add two other corollaries, the one relating to the direction of motion in the line of least resistance, the other to the form of motion, which is always rhythmic, we have, with the principles of the continuousness of motion and of the indestructibility of matter (these representing under two correlative forms the principle of the persistence of force), the sum total of the primary truths which serve as a basis for knowledge in general. But these principles, however general, are only analytical truths; though they are essential to a philosophy, they do not constitute a philosophy. They are the laws of the action of forces separately considered. The universal synthesis which is to constitute philosophy must express the total operation accomplished by the cooperation of these factors. The law which shall formulate this synthesis must be a law of the changes in forces under the two phases, matter and motion, by which they are manifested to us: it must be a principle of dynamics holding good both for the whole of the cosmos, and for its every detail. The changes of an object are all produced by new arrangements of the matter constituting it, and by a new distribution of the forces which belong to it. Their necessary direction is given in evolution in virtue of two principles, both of them corollaries of the primary principle of the persistence of force: the law of the instability of the homogeneous and the law of the multiplication of effects.
Every body tends to pass into a more heterogeneous state, because each of the units that constitute it is of necessity differently affected from the others by the combined action of the others upon it; because the resulting difference places each unit in different relations with the incident forces; finally, because these units, owing to their respective positions, cannot all receive the action of an external force in the same direction and with the same intensity. This law, which accounts for the commencement of the changes, accounts also for its continuance.
At the same time a uniform external force, acting on a body, is there dispersed; acting on unlike parts, it breaks up into forces differing in quality and intensity in proportion to the number and diversity of these parts. The same is to be said of each fraction of the force; the process of dispersion goes on increasing, and the result is expressed by the law of the multiplication of effects.
By another law, flowing from the same primary principle, the parts of a whole diverge from one another in proportion to their diversity, and group themselves together in proportion to their resemblances. Motions that are alike in direction or intensity, acting on these parts, drive them in the same direction, and with the same velocity, whence results an integration of these parts, while those driven by motions unlike in direction or intensity go in different directions with different velocities, separate from one another, are disintegrated. This is the law of segregation, the application of which brings into prominence the heterogeneous character of the products of change, by giving to their heterogeneity a clearer and more definite nature.
Finally, we note another consequence of the persistence of force. Every change in an aggregation of sensible parts is conditioned by opposing forces, the one representing action, the other reaction; the one the tendency to change, the other resistance; their antagonism can end only when equilibrium has been established, by the dissipation of the excess of the one force over the other. A body subject to any disturbance whatever, owing to a modification of its circumstances, tends toward equilibrium with its new circumstances; and, as the different forces acting on it have not the same intensity, those which are weaker soon find their equilibrium, while those which are stronger continue to give motion to the body, and then the latter presents the spectacle of an aggregate whose parts are in an invariable ratio to each other, while the total aggregate is ever changing its relations to external objects. This is equilibrium mobile, unstable equilibrium, and it serves as a transition to a more perfect equilibrium, or else to a renewal of the internal movements which have already found equilibrium.
The action of these laws of change of objects and their parts leads to two contrary results, according to the mode of distribution of the forces in action. We have evolution, i. e., change with integration of matter, dissipation of internal motion, increase of the number and diversity of the parts, whenever the external forces are not such as to break the bond which unites them; we have dissolution, continuous or discontinuous, i. e., a change with disaggregation of matter; absorption of motion (which, becoming internal, drives the constituent units with greater velocity) and diminution both of the numbers and of the diversity of the parts, whenever the external forces are sufficiently intense to destroy the cohesion of the aggregate and to restore to its parts their original independence.
The work of Mr. Spencer in his "Biology" consists in referring to these general laws the generalizations obtained in the various parts of the domain of biology, and in discerning those which possess the character of necessity. This course has the twofold advantage of giving to these generalizations greater authority, and of introducing into a coordinated system of philosophy the science whose generalized truths they are. The "Principles of Biology" is thus an attempt to explain the phenomena called vital, by general laws common to phenomena of every kind.
[To be continued.]