cuckoos' eggs and evolution. 283 



one which has prevailed and its value is cui^iously increased 

 when we find, as we do, that in certain areas both Crows' and 

 Koels' eggs show a certain tendency to a certain type, or degree 

 of colour intensity. Thus the eggs of both Crows and Cuckoos 

 in Assam eastwards average dark ; those from the dry areas of 

 Northern India average paler, whilst those from the dry desei-t 

 ai-eas of Sind, etc., are palest of all. Then when we come to 

 Ceylon and the wet coasts of Malabar and Travancore, we again 

 come back to dark eggs, whilst the drier regions of Ceylon seem 

 to have a special reddish type confined to that island. 



There are, of course, many more similar instances of perfect 

 or complete evolution. For instance, there is Clamator glandarius 

 in (Spain and other parts of Europe which deposits its egg in the 

 nests of Magpies but the examples given are sufficient, I hope, 

 to prove that in some cases certain species of Cuckoos always lay 

 the same type of egg and, normally, always cuckold the same 

 species of foster-parents. 



There is, however, another form of complete evolution which 

 is even more interesting than the last and this is dimorphic 

 evolution, or even, if 1 may use such a word, polymorphic 

 evolution. 



There are some species of Cuckoo which cover a far wider 

 range of country than is covered by any one possible fosterer, 

 and in some instances of this nature we find in their eggs the 

 most wonderful contrasting examples of evolution. 



The two most striking instances of dimorphic evolution known 

 to me are to be found in the eggs of the two Cuckoos, Cucidus 

 poliocejjhalus and Hierococcyx sparveroides. 



Cticuhts poliocephalus is a small Cuckoo which ranges from the 

 "Western Himalayas through the mountains of China to Japan. 

 Over much of her western area she deposits her eggs in the nests 

 of several of the small warblers of the genera Phylloscojyus and 

 Acanthopneuste. All these little birds lay either pure white eggs 

 or eggs which are white, sparsely spotted. Moreover, she more 

 often selects as fosterers those species which lay pure white eggs 

 rather than those which lay spotted ones and her favourites 

 seem to be Acantliopneuste occi2ntalis and A. magnirostris. With 

 these fosterers a white egg is laid. 



In Japan C. poliocephalus is as common as she is in the Hima- 

 layas, but here she selects as fosterer the equally common little 

 Gettia cantans, a species which lays a remarkably beautiful deep 

 terra-cotta or chocolate-pink egg. She seems to deposit her eggs 

 almost invariably in this bird's nest and, accordingly, the egg she 

 lays is similarly coloured, matching extremely well the fosterer's 

 egg in everything but size. I believe the Eev. F. C. R. Jourdain 

 has received specimens of this Cuckoo's eggs with other fosterers 

 but he informs me that these eggs were taken by a Japanese col- 

 lector about Avhom we know little and the late Alan Owston, 

 who sent me over 30 of this Cuckoo's eggs, said he had never 

 known it lay iu anything but nests of Cetiia. Throughout the 



