EVOLUTION OF THE COLORS OF BIRDS. 127 



tion is produced by change in the environmeut, the 

 more effective forms of Selection do not appear till the 

 organism has so multiplied as to produce what I call 

 Superlative Natural Selection through intense competi- 

 tion between rival individuals of the same species in 

 gaining possession of limited resources. And, fifth, 

 that Passive Comparative Natural Selection, which de- 

 pends on change in the environment, without special 

 rivalry between the members of one species, also depends 

 on variation in the adaptations of the organism, many 

 of which variations do not depend on that change in the 

 environment which has produced the change in the 

 Natural Selection, nor, indeed, on any change in the 

 environment except those physical changes by which 

 the world has passed from its primitive gaseous to its 

 present partially liquid and solid state, rendering it a fit 

 abode for organisms." 



He thus entirely ignores the origin of variations and 

 assumes that inasmuch as the environment in the vari- 

 ous forms of selection cited does not produce the varia- 

 tions definitely and directly, it can have no influ- 

 ence whatever in their origination. If the discussion 

 of variation in the preceding pages has any force what- 

 ever, this conclusion certainly should not seem an in- 

 evitable one. 



An attentive consideration of Mr. Gulick's views seems 

 to make one fact undeniable, viz. that some form of 

 isolation is indispensable to divergent evolution. His 

 elaboration of the various forms of isolation is so exten- 

 sive that anything more than a brief outline of it 

 would be impossible in the present connection. This 

 much is, however, necessary to even a general under- 

 standing of the factors of organic evolution. The fol- 

 lowing is Mr. Gulick's classification table of the forms 

 of segregation: 



