HISTORY OF ZOOLOGY 



41 



it is almost certain that in a great measure ttiey have been acquired by the 

 males through the struggle for the female. In the case of birds a second 

 factor has undoubtedly co-operated to impress distinctly the often 

 enormous difference between the feathers of the male and of the female 

 — as is shown, for example, in the case of the birds of paradise (fig. 15) ; 



Fig. 15.A. — Paradisca apoda, male (after Levaillant). 



for the nesting female inconspicuous colors and a close-lying coat of 

 feathers are necessary in order that, undisturbed by enemies, she may 

 devote herself to incubation. 



On the Efficiency of Natural Selection. — In the course of the last 

 twenty-five years there has been much controvers)^ as to how far natural 

 selection alone is a species-forming factor. A number of objectors dispute 

 the possibility of fortuitous variations being utilized in the struggle for 



