Disproportion of Sexes. 155 



of the sexes, marked and permanent, involving such 

 an excess of males as we find among the cuckoos, 

 you have one of the most efficient tendencies to re- 

 duction of the species. The species, then, can only 

 survive by some extraordinary means resorted to by 

 the females — a thing which has exactly happened 

 with the cuckoos, as also, we shall yet see, with the 

 American cow-birds. This extraordinary thing is, 

 in their case, the deposition of eggs in other birds' 

 nests, and the instinct in their young to turn out the 

 eggs or legitimate young from the nests where they 

 may be, to have fullest guarantee they shall be fed 

 and tended. In no case of birds which I have kept 

 in confinement, and have got to breed, is there on the 

 part of the hen any desire for intercourse after one 

 half of her eggs are laid. Most carefully she avoids 

 this, and is wholly unmoved in this direction by any 

 demonstrations of the cock to which he sometimes 

 will yield himself, only then to be firmly put aside by 

 the hen. This is, in conformity with a great law, that 

 after full conception, coition in any form is not bene- 

 ficial to the progeny, or may not be, and in nature 

 the law in this respect is very obtaining — the instinct 

 on the part of the females being absolute in rejecting 

 all sexual advances. The hornbill cock, when he 

 builds in his mate the moment she begins to brood 

 by plastering up the hole of the nest with clay, leav- 

 ing just enough space for his bill to introduce food to 

 her, erects a real barrier to intercourse, but that is 

 only a shadow of a yet more real law. Certain of 

 the ducks — more especially the eider ducks and their 

 allies — when the females begin to brood, depart and 

 take to fresh feeding grounds on the coast or in the 

 straits between the islands. 



