The first volume of the "Principles of Biology" consists of three parts, the first of which sets forth the data of biology, including those general truths of physics and chemistry with which rational biology must start. The second part is allotted to the inductions of biology, or, in other words, to a statement of the leading generalizations which naturalists, physiologists, and comparative anatomists have established. The third and final part of the first volume of the "Principles of Biology" deals with the speculation commonly known as "the development hypothesis," and considers its a priori and a posteriori evidences.

The inductive evidences for the evolutionary hypothesis, as contra-distinguished from the special-creation hypothesis, are dealt with in four chapters. The "Arguments from Classification" are these: Organisms fall into groups within groups; and this is the arrangement which we see results from evolution where it is known to take place. Of these groups within groups, the great or primary ones are the most unlike, the sub-groups are less unlike, the sub-sub-group still less unlike, and so on; and this, too, is a characteristic of groups demonstrably produced by evolution. Moreover, indefiniteness of equivalence among the groups is common to those which we know have been evolved, and to those supposed in the volume before us to have been evolved. There is the further significant fact that divergent groups are allied through their lowest rather than their highest members. Of the "Arguments from Embryology," the first is that, when developing embryos are traced from their common starting-point, and their divergencies and re-divergencies are symbolized by a genealogical tree, there is manifest a general parallelism between the arrangement of its primary, secondary, and tertiary branches, and the arrangement of the divisions and subdivisions of Mr. Spencer's classifications. Nor do the minor deviations from this general parallelism, which look like difficulties, fail on closer observation to furnish additional evidence; since those traits of a common ancestry which embryology reveals are, if modifications have resulted from changed conditions, liable to be disguised in different ways and degrees, in different lines of descendants. Mr. Spencer next considers the "Arguments from Morphology." Apart from those kinships among organisms disclosed by their developmental changes, the kinships which their adult forms show are profoundly significant. The unities of type found under such different externals are inexplicable, except as results of community of descent, with non-community of modification. Again, each organism analyzed apart shows, in the likenesses obscured by unlikenesses of its component parts, a peculiarity which can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. And, once more, the existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, is satisfactorily interpreted by the hypothesis of evolution. Last of the inductive evidences are the "Arguments from Distribution." While the facts of distribution in space are unaccountable as results of designed adaptation of organisms to their habitats, they are accountable as results of the competition of species, and the spread of the more fit into the habitats of the less fit, followed by the changes which new conditions induce. Though the facts of distribution in time are so fragmentary that no positive conclusion can be drawn, yet all of them are reconcilable with the hypothesis of evolution, and some of them yield strong support,--especially the near relationship existing between the living and extinct types in each great geographical area. Thus of these four categories of evidence, each furnishes several arguments which point to the same conclusion. This coincidence would give to the induction a very high degree of probability, even were it not enforced by deduction. As a matter of fact, the conclusion deductively reached is in harmony with the inductive conclusion. Mr. Spencer has deductively shown that, by its lineage and its kindred, the evolution-hypothesis is as closely allied with the proved truths of modern science as is the antagonist hypothesis, that of special creation, with the proved errors of ancient ignorance. He has shown that, instead of being a mere pseud-idea, it admits of elaboration into a definite conception, so showing its legitimacy as an hypothesis. Instead of positing a purely fictitious process, the process which it alleges proves to be one actually going on around us. To which may be added that the evolution-hypothesis presents no radical incongruities from a moral point of view. On the other hand, the special-creation hypothesis is shown to be not even a thinkable hypothesis, and, while thus intellectually illusive, to have moral implications irreconcilable with the professed beliefs of those who hold it.

Passing from the evidence that Evolution has taken place to the question--How has it taken place?--Mr. Spencer finds in known agencies and known processes adequate causes of its phenomena. In astronomic, geologic, and meteorologic changes, ever in progress, ever combining in new and more involved ways, we have a set of inorganic factors to which all organisms are exposed; and in the varying and complicated actions of organisms on one another we have a set of organic factors that alter with increasing rapidity. Thus, speaking generally, all members of the Earth's flora and fauna experience perpetual rearrangements of external forces. Each organic aggregate, whether considered individually or as a continuously existing species, is modified afresh by each fresh distribution of external forces. To its pre-existing differentiations new differentiations are added; and thus that lapse to a more heterogeneous state, which would have a fixed limit were the circumstances fixed, has its limits perpetually removed by the perpetual change of the circumstances. These modifications upon modifications, which result in evolution, structurally considered, are the accompaniments of those functional alterations continually required to re-equilibrate inner with outer actions. That moving equilibrium of inner actions corresponding with outer actions, which constitutes the life of an organism, must either be overthrown by a change in the outer actions or must undergo perturbations that cannot end until there is a readjusted balance of functions and correlative adaptation of structures. But where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions, then the readjustment results through the effects produced on the species as a whole: there is indirect equilibration. By the preservation in successive generations of those whose moving equilibria are less at variance with the requirements, there is produced a changed equilibrium completely in harmony with the requirements.

Even were this the whole of the evidence assignable for the belief that organisms have been gradually evolved, Mr. Spencer holds that the belief would have a warrant higher than is possessed by many beliefs which are regarded as established. As a matter of fact, however, the evidence is far from exhausted. At the outset of the first volume of "Principles of Biology," it was remarked by the author that the phenomena presented by the organic world as a whole cannot be properly dealt with apart from the phenomena presented by each organism in the course of its growth, development, and decay. The interpretation of either class of phenomena implies interpretation of the other, since the two are in reality parts of one process. Hence the validity of any hypothesis respecting the one class of phenomena may be tested by its congruity with phenomena of the other class. In the second volume of "The Principles of Biology," Mr. Spencer passes to the more special phenomena of development, as displayed in the structures and functions of individual organisms. If the hypothesis that plants and animals have been progressively evolved be true, it must furnish us with keys to these special phenomena. Mr. Spencer finds that the hypothesis does this, and by doing it gives numberless additional vouchers for its truth. It is impossible for us here to review, even in outline, the extensive field traversed in the second volume of "Principles of Biology." We would not omit, however, to direct attention to the interesting conclusion reached by Mr. Spencer toward the close of the volume with regard to the future of the human race considered from the viewpoint of the possible pressure of population upon subsistence. He points out that in man all the equilibrations between constitution and conditions, between the structure of society and the nature of its members, between fertility and mortality, advance simultaneously towards a common climax. In approaching an equilibrium between his nature and the ever-varying circumstances of his inorganic environment, and in approaching an equilibrium between his nature and all the requirements of the social state, man is at the same time approaching that lowest limit of fertility at which the equilibrium of population is maintained by the addition of as many infants as there are subtractions by death.