234 COOK 



upon it, have been raised from the time of Darwin to the pres- 

 ent day, but a doctrine with so many merits was not to be dis- 

 placed until another could be found. Furthermore, the alterna- 

 tive views hitherto presented have shared either one or both of the 

 false premises of natural selection, or they are built, like that 

 theory, on some one group of biological phenomena, and leave 

 out of account other data equally pertinent to the general conclu- 

 sion, and equally in need of evolutionary explanation. 



One of the ways in which the search for evolutionary causes 

 went far afield was in assuming a close and essential relation 

 between evolution and the origin of species. It was thought that 

 if it could be known how new species came into existence the 

 secret of the diversity of nature would be revealed. As a mat- 

 ter of fact evolution has very little to do with originating or 

 multiplying species. The evolutionary process continues, we 

 may believe, whether the group becomes divided or not. The 

 two parts become different because evolution continues in both, 

 but it would also have continued if the separation had not taken 

 place. Isolation, of one kind or another, is the cause of the 

 multiplication of species, but not of evolution. We would gain 

 no special advantage for evolutionary observation by stationing 

 ourselves at the point of bifurcation of one group into two ; the 

 only lesson would be that isolation isolates, that segregation 

 segregates. Evolution, it cannot be repeated too often, does not 

 take place in the gaps which are left between the species, but 

 inside of the species, among the interbreeding organisms ; it is 

 an zWrtfspective phenomenon, not interspecific. 



To learn how species differ is only to ascertain what roads 

 they have traveled over, it is only by canvasing the differences 

 between the individuals of a species that we can hope to ascer- 

 tain how the evolutionary progress is accomplished. It will not 

 suffice, when when we find that the individuals of a species differ 

 in a certain respect, to assume that this is the line of evolution- 

 ary advancement. We must be content first to recognize and 

 describe the several kinds of intraspecific differences before we 

 can hope to estimate with confidence the contribution of each 

 form of change to the general and permanent progress of the 

 species. 



