290 ZOOLOGY. 



had not so great an extent, yet they are developed directly, 

 not by gradual increase. The true reading of the matter 

 therefore is not that the ancestral condition is repeated, 

 but that the larval condition of the ancestor is retained, 

 because the larva is still hatched and still lives in the 

 same way ; but the structure after metamorphosis is 

 different, because the fish has acquired different habits. 

 On the theory of mutations we must suppose, not that 

 individuals were hatched which had asymmetrical eyes, but 

 that symmetrical individuals in consequence of mutation 

 became asymmetrical, and there is no evidence of such a 

 mutation having occurred in any fishes which do not lie 

 on the ground on one side. 



15. Secondary Sexual Characters. Another class of 

 zoological facts which cannot be explained on the mutation 

 theory, nor on any selection theory, is that class which 

 concerns what are called secondary sexual characters. It 

 is well known that in a vast number of cases the male 

 possesses structures or colours which are not present in 

 the female: the male and female are different, and the 

 sexes constitute two " forms " in the same species ; hence 

 this condition is called "sexual dimorphism," which means 

 sexual two-formedness. Examples of this are seen in nearly 

 all divisions of the animal kingdom, the cock and hen of 

 the common fowl, the stag and the doe, and the black horny 

 excrescences on the fore-foot of the frog afford familiar 

 examples. Darwin explained such facts by " sexual selec- 

 tion," the males having such variations most developed 

 being either preferred by the female or conquering other 

 males in combat. It is certain that the selection of the 

 male is no explanation, because the variation would be 

 inherited by the female unless it was only capable of 

 inheritance by the same sex, and this must therefore be an 

 original peculiarity of the variation. 



Mutationists explain these characters by supposing that 

 certain mutations are " coupled " with one sex, so that 

 they are only inherited by individuals of that sex. But 

 there is abundant evidence to prove that the characters 

 are really inherited by the female sex, but in a latent con- 



