Ld 
1890.] Probable Causes of Polygamy Among Birds. 1025 
horse, which exhibits slight sexual disparity, are to be found 
among polygamous mammals, but so far as we know, none exist 
among aves. Seeing that such is the case, it is fair to infer that 
monogamous birds should present, if any, much less distinctive 
sexual differences. Generally speaking this is the rule, and it 
will be shown that the exceptions to it result from peculiar condi- 
tions of environment or of physical structure which nullified the 
tendency to polygamy. 
The value of the following remarks must depend somewhat on 
the success with which Darwin has proved that sexual selection 
is the cause of specific distinctions among birds through succes- 
sive variations, induced by the choice of the female during court- 
ship. The conclusions arrived at in this paper are therefore sup- 
plementary to his, and are based on the supposition that although 
he may have attached too great importance to his theory of 
sexual, as distinguished from natural, selection, we must believe, 
nevertheless, that sexual selection exerts a powerful influence in 
the genesis of species. 
Premising this, we may conclude: ıst, That the present status 
of development in birds practicing polygamy is the result of 
sexual selection. 2d, That the standard of female choice was, in 
the majority of cases, the relative perfection of beauty of colora- 
tion in the male and hence also of display by him, from which 
habit would arise the tendency to specialization of form, as in- 
stanced in the extraordinary development of alar and caudal 
appendages among many polygamous species. 3d, That in pro- 
portion to this tendency toward plumage specialization the male 
differs from the female in that he alone develops thus; therefore, 
4th, In view of this, we must conclude that all polygamous species 
have originated from a less ornamented type which more closely 
resembled the female than her mate of to-day, while the difference 
between these was reduced to a minimum in the distant past ; and 
that the young of both sexes remained constant to the garb of 
the adult female during the first year. 5th, That this specializa- 
tion was not transmitted to the female and young, owing to their 
greater need of protection by obscure tints, and to the incon- 
