[89] THE EVOLUTION OF THE FINS OF FISHES. 10G9 



can have no reason to doubt the fact that the energies of the universe 

 are otherwise than continuously exhibited. The organism, like a planet, 

 is bounded by a form ; theoretically it is separated temporarily from 

 the rest of the cosmos just as the former, but it is a parasite, crawl- 

 ing, walking, running, flying, or swimming, on or in the various media 

 found on the surface of the planet. The organism is individualized 

 like the planet, but the energies of boch are reciprocally antagonistic. 

 Action and reaction are equal and continuous in one sense. The en- 

 ergies of the planet and the cosmos, taken together, are, however, 

 too great to be overcome by these superficial i^arasites, and in conse- 

 quence of their small mass they must accommodate themselves, ad- 

 just themselves as the j^ull and push of the vast planet under them 

 determine. They are modified in structure by their own actions and 

 the reactions of the environment, or die if they finl to succeed in effect- 

 ing this adjustment, a j)rocess in which their volition is of small effect 

 in changing the result, because what will they may have must bend 

 and adjust its demands to the demands of far stronger outward forces. 

 The naturalist encounters upon every hand evidences of the effects of 

 the struggle of the organism with its environment. This struggle is so 

 real, so tangible, that it becomes palpable in every movement made by 

 the body which calls for effort in its execution. It is the most contin- 

 uously operative of all the forces of nature, effectual as modifiers of or- 

 ganization. There exists not a single bone known to the osteologist 

 which does not bear the evidence of having been directly or indirectly 

 modified in the course of the conflict between the organism and its en- 

 vironment, in consequence of the interaction between the forces of the 

 latter and those exerted by nature. It will thus be seen that all the 

 forces that are originative, except such advantageous actions as spring 

 from acquired intelligence, are ultimately to be sought in the energies 

 with which an organism has to contend during each and every moment 

 of its existence. The beautiful adaptations which appeal so strongly 

 to the teleologist are only the result of the action of natural forces 

 which originally determined the deposition of inactive, active, sensitive, 

 or contractile materials at definite places in a living body in sufficient 

 proportional amounts to fit it to meet the demands made of it In relation 

 to its environment. 



This is the origin of " fitness," in precisely the same sense in which 

 we may contemplate the origin of the equilibrium existing between 

 the planets. A certain dynamically determined fitness, therefore, pre- 

 cedes the process of natural selection, from which the latter draws its 

 material and upon which heredity fixes its stamp; but heredity, be it 

 remembered, is not a whit more indebted to the action of natural selec- 

 tion than it is to the operation of those far more powerful interacting 

 energies which primarily ordain adaptations. 



The desire to get the most i)leasure with the least exertion is prob- 

 ably a universal character of the habits of organisms, and those which, 



