HISTORY OF ZOOLOGY. 6 1 



peculiar love-dances. Since all these characters belong 

 chiefly to the male, and since it is only exceptionally that 

 they are inherited by the female (and even then less dis- 

 tinctly stamped), 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 undoubtedly a second 

 factor has co-operated to impress distinctly the often enor- 

 mous difference between the feathers of the male and of 

 the female as is shown, for example, in the case of the 

 birds-of-paradise ; for the nesting female inconspicuous 



FIG. 158. Paradisea afoila, female. (After Levaillant.) 



colors and a close-lying coat of feathers will be necessary 

 in order that, undisturbed by enemies, she may devote her- 

 self to the brooding process. 



On the Efficiency of Natural Selection. In the 

 course of the last decade there has been much controversy 

 as to how far natural selection by itself alone is a species- 

 building factor. From many sides the objection has been 

 raised that the variations occurring in nature are at their 

 first appearance generally much too small to be of value to 

 the organism, and thus to become subject to natural selec- 

 tion. Thus, for example, the -.ring of a bird in order to be 

 used for flight must have already reached a size far beyond 



