158 Rev. J. T. Gulick on Divergent Evolution 



environment or that crossing should be prevented. But did 

 he not consider both these conditions necessary for the forma- 

 tion of two or more species from one original species ? 



He says, " Intercrossing will affect those animals most 

 which unite for each birth and wander much, and which do 

 not breed at a very quick rate. Hence with animals of this 

 nature, for instance birds, varieties will generally be confined 

 to different countries ; and this I find to be the case. With 

 hermaphrodite organisms which cross only occasionally, and 

 likewise with animals which unite for each birth but which 

 wander little and can increase at a very rapid rate, a new and 

 improved variety might he quicJdy formed on any one spot, and 

 might there maintain itself in a body and afterward spread, so 

 that the crossing would be chiefly between the individuals of 

 the new variety living together in the same place. . . . 



" Even in the case of animals which breed slowly and 

 unite for each birth, we must not assume that the effects of 

 natural selection will always be immediately overpowered by 

 free intercrossing ; for I can bring a considerable body of 

 facts showing that within the same area varieties of the same 

 animal may long remain distinct, from haunting different 

 stations, from breeding at slightly different seasons, or from 

 varieties of the same kind preferring to pair together. . . . 



" Isolation also is an important element in the changes 

 effected through natural selection. In a confined or isolated 

 area, if not very large, the organic and inorganic conditions of 

 life will be almost uniform ; so that natural selection will tend 

 to modify all the varying individuals of the same species in the 

 same manner. Intercrossing with the inhabitants of the sur- 

 rounding districts will also be prevented. Moritz Wagner 

 has lately published an interesting essay on this subject, and 

 has shown that the service rendered by isolation in preventing 

 crosses between newly formed varieties is probably greater 

 even than I have supposed. But, from reasons already 

 assigned, I can by no means agree with this naturalist that 

 migration and isolation are necessary for the formation of new 

 species," [' Origin of Species,' fifth edition *, Chapter IV., 

 Section on " Circumstances favourable for the Production of 

 New Forms through Natural Selection."] 



Again, in the same chapter, in the section on " Various 

 Objections," in answer to the question " How, on the principle 

 of natural selection, can a variety live side by side with the 

 parent- species ? " he replies, " If both have become fitted for 

 slightly different habits of life or conditions they might live 



# The same passages occur in the sixth edition, pp. 80, 81. 



