Page:Popular Science Monthly Volume 50.djvu/466

448 As here developed in its biological aspects, this law of equilibration deserves the closest attention. Life is defined by Mr. Spencer as "the continuous adjustment of internal relations to external relations"; and he shows that the degree of life varies as the correspondence varies between organism and environment; the highest point being reached where the correspondence exhibits a maximum of complexity, rapidity, and length. Lack of correspondence—that is, inability on the part of an organism to balance external actions by internal actions—means death; absolutely perfect adjustment, on the other hand, would be absolutely perfect life. Now, equilibration, biologically considered, expresses the tendency on the part of an organism to adapt itself to its environment, the environment itself being, it must be remembered, in a state of constant change; and such equilibration is direct where the organism responds immediately to the demands of its surroundings, and indirect where variations which are in the line of greater correspondence are gathered up and transmitted to following generations. Under the one head, it is manifest, we formulate the doctrine of use and disuse; under the other, the doctrine of natural selection. Nor is this all. Followed through its wider sweep of meaning, the law of equilibration is found to throw a flood of fresh light on the vexed question of population. Individuation and genesis are in necessary antagonism; and while "excess of fertility has itself rendered the process of civilization inevitable," the process of civilization must in turn "inevitably diminish fertility, and at last destroy its excess." Gradual approach will thus be made toward an equilibrium "between the number of new individuals produced and the number which survive and propagate."

From The Principles of Biology we pass to The Principles of Psychology, the massive superstructure of which is firmly reared on the general foundations already laid. Life at large is the genus; what we distinguish as bodily life and mental life respectively are species; and though if, after the ordinary fashion, we insist on contemplating only the extreme forms of the two, it would appear that the hardest line of demarcation is to be drawn between them, such line necessarily vanishes the moment the evolutionary point of view is assumed. Acceptance of this point of view, furthermore, enables us to realize that mind can be understood only in the light of its evolution. "If creatures of the