2 1 8 Heredity. 



and pair them without any reference to their own prefer- 

 ences, and continue this for a number of generations, 

 until we have produced a number of divergent races or 

 breeds; if we then find that the males of the^e breeds 

 differ more from each other than the females, we must 

 conclude that there is, behind the action of selection, 

 some more deep-seated law, which determines that males 

 shall, as a rule, be more modified than females. 



Domesticated Pigeons. 



The study of domesticated pigeons is extremely in- 

 teresting in this connection, for it shows conclusively 

 that the tendency which we have shown to exist in 

 nearly all groups of bisexual animals, the tendency of 

 the male to deviate more than the female from the 

 typical structure of allied forms, cannot be attributed 

 exclusively to the fact that the male is more exposed 

 than the female to the action of either sexual or ordinary 

 selection. 



There are more than two hundred wild species of the 

 pigeon family, and throughout the whole, group there 

 is an almost total absence of external difference between 

 the sexes. In a few species the j^lumage is somewhat 

 more brilliantly colored in the male than it is in the 

 female, and it is stated that in one species, Caiyojjliaga 

 oceanica, the excrescence at the base of the beak is a 

 sexual character, but these differences between the sexes 

 are slight and exceptional. 



In domesticated pigeons, on the contrary, the sexes 

 often differ considerably, and it is a remarkable fact that 

 here, as in so many other groups of the animal kingdom, 

 *^ the characteristics of the different breeds are often 

 most strongly displayed in the male bird." (Darwin, 

 Variatio7i, Vol. I. p. 199.) In many cases the sexes are 



