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- 
ception much older in time than the 
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. 
The philosophy 
of evolution. 
