166 



SEXUAL DIMORPHISM 



forms a minute horn on the summit of the flexible appendage, 

 and in a third the whole appendage is converted into a horn. 

 In Darwin's opinion these appendages serve as ornaments. 

 But there can be little doubt that the males fight together 

 with their snouts, and the irritation of the skin of the tip of 

 the snout so produced would result in an outgrowth of the 

 kind described ; if this effect were in some degree hereditary 

 the character would be explained. On the other hand, we 

 have no reason whatever to suppose that such a character, or 

 the successive steps in its evolution, could have arisen apart 

 from the stimulation I have mentioned. 



Unisexual characters are most developed in the chamseleons. 

 In Chamceleon bifurcis and C. Willsii of Madagascar the bones 

 of the face in front of the eyes are prolonged into two great 

 solid projections covered with skin and scales like the rest 

 of the head, and only rudiments of such projections occur 

 in the female. In C. Oivenii of the west coast of Africa, 

 instead of two massive projections there are three slender 

 tapering horns, also bony and covered with smooth skin : 

 of these the female has not a trace. According to Darwin 

 it is not known that these horns are used in fighting, but 

 the animals are known to be quarrelsome, and it is probable 

 that the appendages are thus used. Here again, therefore, 

 we find that the existence of outgrowths corresponds to the 

 impacts produced by definite actions, both being confined 

 to the male sex. It is not stated that the appendages 

 are absent in the young males, but that is probably the 

 truth. 



We find the same exact correspondence in Anolis crista- 

 telhcs. The male is larger than the female, and has a crest 

 along the back and tail which can be erected at pleasure. Of 

 this crest the female does not possess a trace. The males 

 also have a pouch or frill beneath the throat, but this is 

 rudimentary in the female. By direct observation it is 



