1024 The American Naturalist. [November 
PROBABLE CAUSES OF POLYGAMY AMONG BIRDS. 
BY SAMUEL N. RHOADS. 
URING a recent perusal of Darwin’s “Descent of Man,” I was 
impressed by the frequency of his citation of gallinaceous 
birds as best illustrating the theory of sexual selection in its 
relation to the development of secondary sexual characters among 
animals. Probably half the species cited in the four important 
chapters devoted to birds belong to the Gallina, and this may be 
taken as sufficient proof that the order deserves special study 
in our search for the causes of sexual variation, the history 
of descent, and the origin of special characters, which, we have 
reason to suppose, are the result of progressive development from 
ancestral beginnings. 
Gallinaceous birds, as an order, are noteworthy,—nay, almost 
unique,—for their love antics, use of instrumental music to supply 
deficiency of vocal organs, manner of ornamentation in color and 
form, seasonal moult for special protection, combativeness, and 
the practice of polygamy. 
It not being Darwin’s object to treat of polygamy, save in its 
connection with development of secondary sexual characters, we 
find no attempt on his part to explain the causes of it; nor, so 
far as I am able to discover, has such attempt been made public 
by any one. Darwin, however, calls our attention to the fact 
_ that among all avian forms which practice polygamy there are 
none which do not present strongly-marked sexual differences. 
This is significant, and leads to the supposition that the two char- 
acters, being inseparable, are also interdependent. I shall en- 
deavor to prove that polygamy, from the nature of those causes 
which produced it, is necessarily associated with strongly-marked 
sexual differences, though these differences sometimes exist 
among monogamous species; in other language; that distinctive 
sexual characters are a necessary factor to the existence of polyg- 
amy in birds. A few exceptions to this rule, notably of the 
1 Descent of Man, Vol. I., pp. 257-262. 
