46 FOOT-NOTES TO EVOLUTION. 



the naturalist can be guided in his work. If the affini- 

 ties of species are not related to the law of heredity 

 they are unintelligible. If the variation of species is 

 really immutability in disguise we can not trust our 

 senses. It is said, I know not on what authority, that 

 the distinguished ichthyologist, Albert Giinther, was 

 converted to Darwinism by the study of the British sal- 

 mon. Whether this is true or not, such a study could 

 have no other effect. I was brought to the same be- 

 liefs through a study of the minnows and darters of the 

 Mississippi Valley. In the study of species one must 

 choose between some form of development theory on 

 the one hand and a hopeless, unscientific, impossible 

 ignorance on the other ; and in all forms of biological 

 investigation, comparative anatomy, morphology, em- 

 bryology, histology, we reach the same choice of alter- 

 natives. 



The theory of descent by " natural selection " has 

 become in the hands of Herbert Spencer a part of a 



general philosophy of evolution, a con- 

 The philosophy ^ rauch o]der in ^ ^ ^ 



of evolution. . . 



theory of Darwinism. Manifestly we 



could not imagine a homogeneous universe or a homo- 

 geneous earth which could perpetually retain a homo- 

 geneous condition. A cooling earth must lose its per- 

 fect rotundity, its surface must become diversified, and 

 its relation to the sun must cause its equatorial portion 

 to become different from its poles. A single homogene- 

 ous form of life could not remain single and uniform, 

 because life must respond to the conditions of its envi- 

 ronment. Any organism under a tropical sun is not 

 what it would be, exposed to arctic cold. Diversity once 

 begun, and a rate of increase more rapid than a limited 

 earth could permit unchecked, the natural competition 

 in the struggle for existence accounts for the rest. 



