105 [Vol. xlii. 



of eggs suitable for very diflferent types of fosterer which 

 are admirable for their purpose, yet, on the other hand, we 

 have the Common English Cuckoo parasitic on a very large 

 number of birds and with no very striking type of egg. 



I hope, and indeed believe, that the series I will show on the 

 15th prox. will prove that evolution in the Cuckoo's egg is 

 beyond dispute, and that in some cases it is practically com- 

 pleted, in others far advanced, in some backward, and in a few 

 very incomplete. Complete evolution has to be complete in 

 two ways : first, the Cuckoo must have learnt to cuckold a 

 restricted class of fosterer, and, secondly, the type of egg 

 evolved must be of such a nature as to simulate the one or 

 more types of fosterers' eggs with which it has to be deposited. 

 The two are, of course, entirely interdependent, and un- 

 doubtedly the evolution of all Cuckoos' eggs proceeds by the 

 foster-parents refusing to hatch those eggs which are least 

 like their own. In this way those strains of Cuckoos which 

 laj-- the most unsuitable eggs are gradually eliminated, and 

 this process continues until a stable type, or types, of eggs 

 are produced which are sufficiently like those of the selected 

 fosterer to defy normal detection. 



Foremost, as examples of complete evolution, I would draw 

 your attention to the following : — 



Clamator jacohinus. The normal foster-parent of this 

 Cuckoo is any Babbler of the genera Turdoides and Argya, 

 the species or race of these varying according to the locality. 

 But all alike lay blue eggs of almost exactly the same shade of 

 blue as that of the Cuckoo's egg ; indeed, it is often hard for 

 a collector to tell, by looks alone, a bright egg of the Cuckoo 

 from a dull one of the Babbler. So common is this Cuckoo 

 over many parts of India that we often see two or more of 

 their eggs in the same nest, and in one set exhibited there 

 are six Cuckoos' eggs and three Babblers', and you will see 

 that those of the Cuckoos are plainly laid by fewer than 

 three birds. Perhaps it is due to its being so common a^nd 

 the evolution of its eggs so complete that this Cuckoo, 

 usually a bird of the plains, is now widening its field and 

 visiting mountainous country in which its normal fosterers 



