JgiSb] by Birds of Eggs unlike their oiviu 145 



had found an equivalent here) through such species as 

 Pycnonotus layardi, individuals of which discriminated 

 closely, others less closely and one or two not at all^ to 

 such a species as Hyphantornis Jamesonl, which, in my 

 experiments, ejected or destroyed all eggs that were appre- 

 ciably unlike its own. It is even possible that the Hedge- 

 Sparrow may be a recent victim and the Redstart an old 

 one, and the transition between them as much a matter of 

 past selection as of any original difference in discriminating 

 power between the species concerned — not that this will 

 not, in many cases, have existed. 



With the growth of discrimination on the part of the 

 species most victimized — ^and special victimization would be 

 a matter both of abundance and (through natural selection 

 and correct choice of other survivors) of suitable feeding — 

 would come mimicry. I doubt whether this would always 

 end the matter, for, when a Cuckoo's egg became indistin- 

 guishable from its host^s, variation in the latter would still 

 afford the means of distinguishing it from the Cuckoo's, and 

 it is even imaginable that a race may in some cases have 

 taken place between the host's eggs and those of the over- 

 taking Cuckoo. High distinctiveness might sometimes have 

 been the result. In other cases sheer variability would help 

 much to bafHe the Cuckoo whatever its choice were founded 

 on, and useful polymorphism, as in the eggs of the heteroic 

 Warblers and Weavers, might even be selected, and the 

 influence of parasitic birds have thus contributed much, in 

 the course of ages, towards the production of that quality 

 of diversity that to-day so characterizes Passerine eggs. 

 It will not have been the only factor, for the possibility of 

 preference remains, and the actual stimulus to variation will 

 doubtless always have been environmental. Experiments 

 in this last connection might have very interesting results^ 



The similar diversity that is found in the eggs of Cuckoos 

 has been sufficiently explained by other observers. I am 

 not inclined to regard homoism (if the word is permissible) 

 as necessarily more recent than heteroism in Cuckoos' eggs. 



SEH. X. VOL. VI. L 



