1 91 8.] by Birds of Eggs unlike their own. 143 



We start with a polygamous species, with several females 

 laying in the same nest, as in Crotophaga. The male or 

 first-incubating females drive away late layers or the nest 

 becomes over-full. The layers then go off and either lay in 

 other nests of the same species, not yet full, till the same 

 thing happens there, or, nests failing, drop their eggs about 

 and waste them. In both the earlier and the later nests 

 Darwin's suggested advantage — undelayed incubation of 

 eggs laid at nearly the same date— comes about. Darwin 

 laid stress on this advantage in relation to the Rheas, it 

 being his own theory here, and strongly endorsed the view 

 of " some naturalists " that parasitism on unrelated species 

 would confer a similar advantage ; but it may be said, 

 I think, that the advantage would have already been present 

 in the stage thus described — the stage reached by tlie 

 Rheas, — and that parasitism, therefore, would confer not 

 so much this as a further advantage. For where, instead of 

 wasting their eggs, the layers placed them, nests of their 

 own species being no longer available, in nests of other 

 species (a very natural development), a certain proportion 

 of them would be saved : perhaps a very large proportion 

 when the habit first arose, if it be true that selection has 

 had much to do with the perfecting of the qualities of sus- 

 picion and discrimination in hosts, and if overcrowding of 

 the foster-nests were either not serious or were eliminated 

 early by selection. The hens that became broody last, or at 

 least, perhaps through laying most eggs, would tend to be 

 the chief layers in strangers' nests, and the loss of the desire 

 to brood, being now correlated with a habit that brought 

 with it all the advantage between probable survival and 

 certain elimination, might become accentuated in succeeding 

 generations through the action of natural selection. 



A point to be borne in mind, T think, is that a primary 

 necessity throughout will have been that of obtaining the 

 right food for the nestling and that the latter may not have 

 been so well adapted at the outset to a somewhat varying 

 diet as it perhaps is now. It may be the case (and this 

 could be tested experimentally and by stomach-examination) 



