224 SEXUAL DIMORPHISM 



which, as often happens, the male peculiarities were to some 

 degree developed. Mr. Holt found that the largest females 

 generally presented a slight elongation of the anterior rays, 

 accompanied by the black spot bordered with red, and we 

 know that such a modification of the female in advanced age 

 is common enough in dimorphic species. The female Coris 

 in which this was observed were from 11*5 to 14 cm. long. 

 The male or royale form attains to a length of 16 cm. 



But of course the real reason for regarding these two 

 forms as belonging to one species is that the characters to 

 which the controversy refers only occur in specimens above 

 a certain size. Mature males of both forms occur, but small 

 immature males of the more conspicuous form do not. Mr. 

 Holt states that no specimens of the royale have been seen less 

 than 9 cm. long. 



We are forced to conclude that males of the one form are 

 transformed into the other, and that a certain amount of 

 modification may occur in the females. The assumption of 

 the adult characters may not occur in all specimens at the 

 same size, both because the rate of growth varies, and because 

 puberty may not always occur at the same age. But there 

 can be no reasonable doubt that the two forms of Coris are 

 the males and females of one species, the young closely 

 resembling the adult female. The more specialised form is 

 produced by the modification of the other, not from a distinct 

 parentage. 



Of the habits of the fish nothing seems to have been 

 recorded which could be regarded as physiologically related 

 to the evolution of the unisexual characters of the male. 

 Day quotes the observation of Birchell (Zool. 1876) that the 

 fish are accustomed in an aquarium to hide in a bed of shingle. 

 It is probable enough that, when breeding, fishes of this species 

 practise a courtship similar to that observed by Saville Kent 

 in Zabrus mixtus. In this case the sexual excitement of the 



