CHAPTER IV 



FISHES 



In considering the sexual dimorphism of birds or mammals 

 we are dealing with a number of species in all of which 

 (omitting in mammals the Monotremata, which are peculiar) 

 the method of reproduction is the same. In mammals the 

 male is provided with a penis entirely separate from the 

 rectum, and by means of this organ the semen is introduced 

 into the female uterus. The fertilised ova develop within 

 the uterus, and the young are born alive. In birds the penis 

 is less specialised, but fertilisation is likewise internal. The 

 fertilised ova, however, do not develop within the body of 

 the mother, they are provided with yolk, albumen and shell, 

 and then extruded. The great diversity that exists in both 

 these classes with regard to the specialisation of unisexual 

 characters shows that the degree of this specialisation does 

 not correspond to peculiarities in the method of reproduction 

 or fertilisation. 



When we consider the various kinds of fishes we find 

 great diversity in the methods of reproduction, and at the 

 same time less diversity in the specialisation of unisexual 

 characters than in the case of birds. In fishes sexual 

 dimorphism is not necessarily associated with any one 

 method of reproduction. The chief aim of this work is to 

 point out that the degree of sexual dimorphism corresponds 



