448 POPULAR SCIENCE MONTHLY, 



As here developed in its biological aspects, this law of equili- 

 bration * 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 ex- 

 hibits 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 ; abso- 

 lutely 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 en- 

 vironment, 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 fol- 

 lowing generations. Under the one head, it is manifest, we formu- 

 late 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. Indi- 

 viduation 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 ap- 

 proach will thus be made toward an equilibrium "between the 

 number of new individuals produced and the number which sur- 

 vive 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 respect- 

 ively 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 un- 

 derstood only in the light of its evolution. " If creatures of the 



* The general law'is worked out in full in First Principles (Part II, chapter xxii). The 

 question is there raised, Can the changes constituting evolution go on without limit? And 

 the answer is. No. " The changes go on until there is reached an equilibrium between the 

 forces which all parts of the aggregate are exposed to, and the forces these parts oppose 

 to them." 



f Principles of Biology, 376. % Itid-> ^V?. 



