109 [Vol. xlii. 



dimorphic. In grornid-colour tliey are pale bine or pure white, 

 and thev have spots, freckles, or t-mall blotches, varying a good 

 deal in number, size, and depth of colour. They are parasitic 

 on small birds of the genera Suya, Cisticola, Ortkotomus, 

 Franklinia, etc., and all these birds lay eggs which are 

 polymorphic. At present we do not know enough to bo 

 able to decide whether the Cuckoos have had evolved for 

 them dimorphic eggs so as to give a fair chance of a certain 

 proportion finding a home in a suitable nest, or whether the 

 fosterers have become polymorphic in their egg-layino- in 

 order that they may have an average chance of discrimi- 

 nating between their own and the stranger's Qgg, and so 

 allowing a sufficient percentage of their offspring to survive 

 and carry on the race. Swynnerton's remarks on this 

 subject in 'The Ibis' for 1914 are most interesting. In 

 Box No. 25 we may have the key to this question,, for 

 there is here exhibited the most wonderful elimination of 

 the unfit egg in favour of the Cuckoo. In Hyderabad, 

 Deccan, the most common form of small bird suitable as a 

 fosterer for Cacomantis is Prinia socialis, which lays bright 

 brick-red eggs. By the process of elimination a red egg 

 has been evolved for the Cuckoo which agrees sufficiently 

 well with that of the Prinia to ensure a sufficient number 

 being accepted and reared by that bird. In this box are 

 shown two series — one belonging to Professor Burnett of 

 the Hyderabad College and lent to me for this occasion, 

 and the second mostly collected by that gentleman and 

 Col. Sparrow and given to me. These show almost all 

 grades of colour from the typical white and blue eggs to 

 the absolutely atypical red form. You will see that there 

 is a distinct gap between the ordinary blue and white eggs 

 and those most like them amongst the red eggs. This 

 would seem to imply that the total destruction of the least 

 protectively coloured eggs has already gone a long way, 

 although many generations will still have to come and go 

 before the red Cacomantis egg has arrived at the permanent 

 and perfect stage acquired by the eggs of Clamator jacobinus 

 etc. It is interesting to know that this little Cuckoo also 



