DARWIN. 183 



And for this change we have to thank Darwin. 

 ** It is easy to plough when the field is cleared;" 

 and what he first saw clearly, we cannot fail to see 

 now. The fact is, that every student of species and 

 of the facts of geographical distribution before or 

 after Darwin, has reached willingly or unwillingly 

 the conclusion that species are not immutable; 

 that those differences by which he tried to discrim- 

 inate the groups of organisms which he called spe- 

 cies were not differences originating in the act of 

 creation, but produced in some way by outside 

 influences, or by the reaction in the organism from 

 adjustment to these influences. One might safely 

 pledge himself to convert to some phase of the 

 development theory any honest and intelligent man, 

 whatever his preconceived opinions, who would 

 spend a month in the careful study of a large 

 collection of specimens in any group of Natural 

 History, in which the existing species are found 

 over wide areas on the surface of the earth. The 

 study of squirrels, eels, cat-fishes, cray-fishes, pine- 

 trees, asters, butterflies, clams, snails, horses, or 

 men, — any of these will serve perfectly to accom- 

 plish this purpose. 



The general acceptance of the Darwinian theory 

 by naturalists is not due exclusively to the influence 

 of the '' Origin of Species," or to any of the nu- 

 merous commentaries and expositions which have 

 come from other hands. It arises from the results 

 of the studies themselves. The idea of develop- 

 ment gives the only clew by which the naturalist 

 can be guided in his work. If the affinities of 

 species are not related to the law of heredity, they 



