AMERICAN ZOOLOGISTS AND EVOLUTION. 5 



five in number, namely: a germ, nutritive matter, air, water, heat, 

 the four latter undoubtedly existing in the interior of all animals." 1 

 Dr. Leidy affirms his belief that very slight modifications of these essen- 

 tial conditions of life were sufficient to produce the vast variety of liv- 

 ing beings upon the globe. The theory of derivation based upon the 

 principles of natural selection demands the following admissions : that 

 species vary, that peculiarities are transmitted or inherited, that a 

 greater number of individuals perish than survive, and that the physi- 

 cal features of the earth are now and have been constantly changing, 

 and that precisely the same conditions never recur. These are ad- 

 mitted facts. Now comes the theoretical part of natural selection, 

 namely, that the varieties which survive are those which are more in 

 harmony with the environments of the time. These propositions, 

 with minor ones, form the theory of Darwin. Lamarck and others 

 had recognized the gradual enhancement of varieties into species, but 

 had not struck the key-note of natural selection, though Wells in the 

 beginning of the century had clearly recognized it in a pertinent 

 example. If we look impartially at these propositions, we need no 

 demonstration to prove the inheritance of characters the most minute, 

 and even the perpetuation of the most subtile features. 



On general principles, too, the proposition, that those individuals 

 best adapted to their surroundings survive, need only be stated to be 

 accepted by a reasonable mind. In truth, to deny it would be to 

 deny, as Alphonse de Candolle says, that a round stone would roll 

 down-hill faster and farther than a flat one. Indeed, this eminent 

 botanist affirms that natural selection is neither a theory nor an hy- 

 pothesis, but the explanation of a necessary fact. The constant physi- 

 cal changes in the past and present condition of the world are incon- 

 trovertibly established. It seems, then, that the prime question re- 

 solves itself into whether each species as a whole has something 

 inherent which prompts it to vary irrespective of its environments, or 

 whether a correlation can be established between the variation of spe- 

 cies and certain physical conditions inducing these variations, and 

 here let me add that of all groups of animals from species through 

 genera to higher divisions, that group of individuals recognized as a 

 species has the most tangible existence. And, as a proof of this, 

 there need only be mentioned the fact that many naturalists, while 

 regarding species as clearly distinct, have on the other hand looked 

 upon classification as an artificial method to facilitate the study, and 

 hence the innumerable schemes and the successive interpolation of sub- 

 classes, sub-orders, sub-families, and sub-genera, which simply circum- 

 scribed smaller proofs than had before been recognized. 



The rapid multiplication of some of these groups has already 

 formed a serious obstacle to the study of systematic zoology. 



What would good Dr. Mitchell have said if he could have foreseen 



1 " Proceedings of the Philosophical Academy," vol. iii., p. 7. 



