282 , MR. E. C. STUART BAKER ON 



eye. In the third case, although we have an excellent and 

 apparently sufficient simulation of the colour of the Crow's egg 

 by the Koel's egg, yet any child could tell the difterence between 

 the two. 



After reading the above, it will doubtless be asked whether 

 the Cuckoos referred to do not ever deposit their eggs in birds' 

 nests other than their normal fosterers. 



As regards the first two birds, H. varius and C. jacohinus,, 

 certainly one hundred eggs will be found in the nests of Argya 

 and T'urdoides to every one in any other bird's nest. 



GlaTnator coroniandus lays' at least ten times in the nests of 

 G. pectoralis or G. moniliger to every once in other birds' nests 

 and even then she generall}'- deposits her egg in the nest of some 

 other Garrulax, Dn/onastes, or Trochalopteruvi, which ai-e very 

 closely allied birds and which lay eggs either very much like, or 

 rather like, those of the proper fosterers. 



The Koel may be said never to lay in any nest but in that of 

 the true Crows. The only exceptions to the many thousands of 

 eggs recorded are a few deposited in the nests of Magpies in 

 Northern Burma and West China and one instance of its being 

 laid in a Myna's (Acridothet-es tristis) nest. 



We ma}^ say, therefore, in the three examples given, the first 

 including two species of Cuckoo, we have eggs which have 

 arrived at a point in their evolution which is perfect or complete. 

 Even if not perfect in human eyes, the adaptation in resemblance 

 between the eggs of the Cuckoo and foster-parent is so great 

 that a single type of the latter suffices over the whole range of 

 the former to ensure a sufficient number of young Cuckoos being 

 annually reared to replace normal loss. Further, the resemblance 

 is sufficiently perfect to ensure deception of the fosterers without 

 there being any necessity for variation in the Cuckoo's egg. 



The above may, I think, be taken also as examples of wholly 

 stabilized evolution, but it is perhaps advisable to explain that 

 even this stability varies somewhat in degree. Thus in the eggs 

 of both the Crested Cuckoos and in Hierococcyx varius the colour 

 is always the same and varies only very slightly in depth of tint. 

 In the Koel, however, the variation is gi-eater and, in fact, in 

 this respect it may even be an advance in evolution over those of 

 the others, for the variation in tint of colour, of extent of 

 marking, etc., may all be evolved in order to give the possessor a 

 better chance of escaping rejection. 



All Crows' eggs vary greatly in colour ; some are pale, some 

 dark ; some greener, some browner ; in some the reddish 

 markings predominate, in others they are almost absent. To 

 suit such variation in colour there are two possible types of 

 evolution which would occur to us as most useful : first, the pro- 

 duction of an average coloured egg which would agree with an 

 average number of Crows' eggs ; secondly, a similar variation in 

 degree of coloration in both Crows' and Cuckoos' eggs, so that an 

 average number of eggs would match. The latter scheme is the 



