cuckoos' eggs and evolution. 285 



and the Shortwing are very like those of the Cuckoo, the former 

 more especially. 



The second type of egg is a bright pale blue, similar in other 

 respects to the brown egg, but much bigger. This egg is deposited 

 in the nests of the Laughing Thrushes which lay eggs of a 

 similar, sometimes almost indistinguishable, type to those of the 

 Hawk Cuckoo. Another favourite fosterer is the Whistling 

 Thrush, Myiophoneus Ti. temminckii, which lays eggs very like 

 pale, washed-out specimens of those of blackbirds. 



Now the blue eggs are laid by this Hawk Cuckoo all over its 

 range in the Himalayas from the North-West, where they were 

 taken by Col. Rattray, A. E, Osmaston, and others, right away 

 to the Burmese Hills, where a great number have been taken 

 by Messrs. K. Macdonald, J. M. D. Mackenzie, C. Hopwood, 

 P. Wickham, and others. The brown type, on the other hand, 

 appears to occur only in Assam, and even there practically only 

 in the hills south and east of the Brahmaputra. The reason, 

 however, does not require much searching for, as it is only in 

 this tract of country that the Spider-Hunter occurs in sufficiently 

 large numbers to ensure a constant supply of fosterers. Even 

 in Assam, however, the blue-egg laying Cuckoo is almost as 

 common as the other, so that here we get about an equal number 

 of both eggs. There is, in spite of this, no intermediate form of 

 egg in this area, nor do we ever find a blue egg in a Spider- 

 Hunter's nest or a brown egg in that of a Laughing Thrush. 



The last two examples are, as I have already pointed out, not 

 quite parallel. C. poliocephalus lays two contrasting types of 

 egg. In each extreme of its habitat only one form is laid, 

 agreeing closel}" with the one type of fosterer selected but in the 

 centre of this Cuckoo's habitat the two types of egg occur in 

 equal numbers and the process of elimination has not yet arrived 

 at a point where it has succeeded in wiping out all individuals 

 which lay their eggs in wrong nests. H. sparveroides has 

 advanced a stage further ; it also lays two strongly contrasting 

 types of egg, but it lays one type of egg which is blue from 

 extreme east to extreme west of its habitat, whilst in one com- 

 paratively small district, only some 700 miles long, it lays a 

 second brown type of egg which has been evolved to suit 

 exclusively one particular fosterer, which is sufficiently numerous 

 and, presumably, exceptionally suitable in other respects. 



In both the above examples evolution seems to have advanced 

 to a Stage of completion or stability in regard to coloration and 

 size of egg but in the one, G. poliocephalus, there still remains a 

 complete degree of stability to be acquired in the selection of 

 fosterers over a portion of the area inhabited. I should, how- 

 ever, say that, even in this instance, I personally place little 

 importance on the fact that numerous eggs are laid in the nests 

 of more or less abnormal fosterers, as this can be accounted for 

 in many rational ways. This point I deal with more fully later 

 on. 



