164 Darwin and Romanes dealt with. 



game-cock is proverbial, he said, by way of short 

 iUustration. Now, this points the way to a pecuHar 

 physiological fact : polygamy in the lower creatures 

 tends to develop the sexual organs of the male, 

 whereas polyandry among them tends to arrest the 

 development of these organs — to lessen them and to 

 cause after long lapse of time, a kind of shrinking 

 away. This is most decidedly the case with our 

 cuckoos ; it is, so far as I can learn, the case with the 

 American and other foreign cuckoos ; it is the case 

 with all varieties of the Molothrus species. It is the 

 case with certain worms and insects, according to 

 Professor Semper and Professor Van Beneden, as we 

 have just seen. The polyandrous birds do not, so far 

 as I can find out, indulge in the combats that the 

 polygamous birds more especially tend to do ; the re- 

 duction of the organs allaying passion and making 

 them content to share favours ; suggesting the ques- 

 tion whether in the case of birds, the males of which 

 do now still fight, they had not at one time been poly- 

 gamous, and that changes and influence of a special 

 kind had not so equahsed the sexes as to make poly- 

 gamy longer impossible. We know, for example, 

 that ground-sitting females are more exposed to 

 danger of various kinds than the males are : and this 

 may be an element in it : the change of nest either in 

 place or in form is largely due to the risk and danger 

 that had arisen on the sitting bird, as in the case 

 clearly of some of the weaver-birds and others we 

 might refer to. The almost apparently arbitrary 

 adoption of domed nests by magpies and jays may 

 also have to do with this — the prevention of dispro- 

 portion in the sexes. 



