570 Mr. C. F. M. Swynnerton on 



of the Weavers, and the apparent view of observers that 

 gregariously-breeding sea-birds also recognize their nests by 

 position, with the indirect evidence perhaps supplied by the 

 fact that eggs of the Common Guillemot of the same 

 coloration would seem to have been found in exactly 

 the same spot in successive years, tells against it even here. 

 Extensive experimental transposition of eggs will quickly 

 settle the question. 



The question, of course, presents itself: Are fliese neces- 

 sities, then, of greater importance than that of ready recog- 

 nition by enemies ? Probably, where nauseousness exists, 

 they are not, but in any case it is rare, I think, to find that 

 where two necessities conflict one merely prevails over the 

 other. Both have to be adequately dealt with, and usually 

 counteragents are found to be present, by means of which 

 one necessity is reduced or both fully met. In the cases 

 under discussion both these methods seem to be in operation. 

 Thus the absence of nauseousness, the fighting qualities of 

 the Drongo and certain other conditions probably act as 

 "reducing'^ counteragents in the way I shall suggest below, 

 while in some of the Weavers the " enemy ■" demands are 

 themselves, I believe, boldly met by mimicry, engrafted 

 on to a polymorphism that may have been primarily selected 

 in relation to the baffling of Cuckoos. Again, even if it 

 originated in a common ancestry, the fact that the poly- 

 morphic eggs of different species of Weavers and (at any rate 

 at Chirinda) of different genera of Warblers, tend to run to 

 similar forms in the same locality, ensures an ample popu- 

 lation and an ample resulting notoriety for each form in 

 relation to enemies, while each species still enjoys the full 

 advantage given it by polymorphism in relation to Cuckoos 

 and (if that be a necessity) to parental recognition. At the 

 very worst (if the Cuckoo does try to match her egg — which 

 at present seems unlikely — and is the better enabled to do 

 so through the larger population of each form) the risk is 

 divided up between two or more species. I may mention, 

 finally, the constancy (in my own limited experience) of the 

 non-mimetic forms of polymorphic eggs, amounting in some 



