100 PECKHAM. [Vol. 1, 



In at least two genera of the Gasteracanthidse, then, there 

 is a difference between the sexes, which is the result of the 

 inheritance of special protective modifications by the female 

 alone. 



In replying to Wallace's theory that color differences 

 between the sexes are due to the keeping down for protective 

 purposes of the color of the female, Darwin says: "I do not 

 wish to deny that the female alone of some species may have 

 been specially modified for protection."* In most cases when 

 the female is dull and the male bright colored it is difficult or 

 impossible to show that the difference has resulted from the 

 greater need of protection in the female. The difference in the 

 Gasteracanthidse, however, is just such a case as was referred 

 to by Darwin, since it is unquestionably due to natural and not 

 to sexual selection. 



The young of Gasteracantha and Acrosoma present certain 

 pecularities. Darwin's propositions in regard to differences 

 between the two sexes in birds are as follows: 



"Whenever and in whatever manner the adult male differs 

 from the adult female, he differs in the same manner from the 

 young of both sexes. * * * 



"When, on the other hand, the adult male closely resem- 

 bles the young of both sexes (these, with rare exceptions, being 

 alike), he generally resembles the adult female."t 



Acrosoma and Gasteracantha are not included in these 

 rules, since although the young are alike at the very first, both 

 resembling the adult male, the young females very early begin 

 to vary toward the form and color of the adult female ; so that 

 we have the young males and females resembling the adults of 

 the same sex, while the adult male and female differ greatly 

 from each other. 



Darwin notes the occasional occurrence of such cases as 

 this in the following words: 



"As variations occurring late in life, and transmitted to 



*Descent of Man, p. 321. 

 iLoc. c»(.,pp. 232, 233. 



