cuckoos' eggs and evolution. 281 



texture but also in shape for, whereas the Laughing Thrushes 

 normally lay i-ather long, oval-shaped eggs, this Cuckoo lays the 

 most spherical of all eggs in the family. 



This is rather interesting, for we find that not only is a 

 spherical egg a very common form in reptiles, but that the 

 more primitive non-parasitic Cuckoos also often lay spherical 

 eggs. It would rather seem, therefore, as if shape was not a 

 characteristic of much importance in birds' eyes, a theory which 

 is confirmed by the difference in shape obtaining between other 

 Cuckoos' eggs and those of their fosterers. Probably we may 

 take this as a case of " complete evolution " if not of " perfect 

 evolution," as all that is necessary has been achieved by the 

 production of an egg similar in size and colour to that of the 

 fosterer. 



Yet another instance of complete, though not perhaps perfect, 

 evolution may be found in the eggs of the Koel, Eudynamis 

 scolopaceus, which lays its eggs in the nest of various species of 

 the Gorvidce. Here we have birds, looked upon as the most 

 astute of all the Passerine birds, regularly duped by a smaller 

 bird, although everyone who knows India and who has taken 

 the most superficial interest in birds, is fully aware that the 

 Crow, Avhatever his species or race, has the bitterest instinctive 

 hatred of the Koel. 



The Koel is a large bird, and really belongs to a section of the 

 Cuckoo family, the other members of which are non-parasitic. 

 She naturally requires fosterers who are in a position to obtain 

 a very large quantity of animal food for the voracious youngsters 

 and, probably, in the area she occupies, the Crow is the only bird 

 found from Ceylon to the Malay Archipelago whose habitat is 

 as widespread as her own and who could furnish such food. 



Accordingly we find that an egg has been evolved by the 

 process of elimination sufiiciently perfect for the purpose of 

 hoodwinking the Crows and ensuring perpetuation of the 

 Cuckoo. Crows' eggs are too well known to need much descrip- 

 tion but, roughly, they are pale bluish in ground-colour, with 

 profuse blotching of dull red all over their surface, the 

 general efiect of the colour-scheme being a dark greenish egg. 

 The Cuckoo's egg has been improved and perfected, until we get 

 a type very similar in general colour to that of a Crow's egg 

 but it is not alike in shape, for it is generally a shorter, blunter 

 oval and in size it is much smaller. 



The above three examples may all be said to be instances of 

 complete evolution but I purposely use the term perfect evolu- 

 tion only in reference to the first two birds' eggs (C. jacobinus 

 and If. varius). Here we have eggs so close to those of their 

 foster-parent in colour, shape and size that often human eye- 

 sight is not acute enough to distinguish between the two. In 

 the second case, the eggs of the Eed-winged Crested Cuckoo and 

 those of the Laughing Thrushes are very alike in colour and also 

 in bulk, though- their shape betrays them at once to the human 



