EVOLUTION OF THE COLORS OF BIRDS. 131 



gether with the greater rapidity and power with which 

 the sexual elements of the same species, race, or indi- 

 vidual combine, as contrasted with the rapidity and 

 power with which the elements of different sj)ecies, 

 races, or individuals combine." 



17. Segregate Fecundity. Relation of species or 

 varieties inter se, when free crossing between " members 

 of the same species or variety results in higher fertility 

 than the crossing of different species or varieties." 



18. Segregate Vigor. "Relation in which species 

 or varieties stand to each other when the intergenera- 

 tion of members of the same species or variety produces 

 offspring more vigorous than those produced by crossing 

 with other species or varieties." 



(c) Institutional Segregations. " Produced by the 

 rational purposes of man embodied in institutions that 

 prevent free intergeneration between the different parts 

 of the same race." 



C. Intensive Segregation. When two or more 

 sections of a species are mutually isolated by any of 

 the above forms of segregation and are subject to the in- 

 fluences of a highly complex transforming agency, there 

 is a reasonable certainty that the different divisions will 

 be differently acted upon. In succeeding generations 

 there will be increasing degrees of divergence and seg- 

 regation, which is termed the law of Intensive Segrega- 

 tion. The different forms of intension are due to 

 difference in food, influence of the environment, use 

 and disuse, correlation of growth, selection, etc. In 

 this case, it will be noticed that Mr. Gulick takes due 

 notice of the influence of the environment, despite his 

 assertion that variations may, in many instances, be 

 wholly independent of it. 



It is time to bring this introduction to a close 

 and hasten on to the more special considerations for 



