HISTORY OF ZOOLOGY -11 



it is almost certain that in a great measure they 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. i5A. Paradisea 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 controversy 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 



