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.

V.