1918.] Erythrism in Birds' Eggs. 75 



hands, I have never seen an egg intermediate in colour 

 between the two types — a fact which shows that evolution 

 has advanced practically to completion. The white eggs 

 are, of course, deposited in the nests of birds which lay 

 white eggs or eggs which are not too vividly spotted upon a 

 white ground, such as the Phylloscopi and Acanthopneuste ; 

 the red eggs, on the other hand, are placed in the nests of 

 such birds as Horornis, Cettia, Oligura, Tesia, etc., which lay 

 eggs very similar in colour. Over the greater part of its 

 range both types of eggs of this Cuckoo may be found, for 

 suitable fosterers also occur everywhere therein ; but in 

 some places one type will preponderate greatly over the 

 other, according as the white-egg or the red-egg fosterer is 

 the more numerous. In Japan, however, so far as I am 

 aware, the terra-cotta type of egg is the only one ever laid, 

 and that is simply because the Cettia, in whose nest it is 

 placed, is so common and has proved so good a fosterer that 

 it has not been necessary to select any other, and the Cuckoo 

 practically invariably places her egg in that bird's nest. 



The last group contains eggs of the Indian Plaintive 

 Cuckoo, Caccomantis passerinus. This Cuckoo, over nearly 

 the whole of the area in wliich it breeds, lays eggs which 

 are either a pale blue or white, faintly and sparsely marked 

 with reddish ; these are deposited in the nests of Orthotomus 

 svtorius^ Cisticola, Suya, or some other small Warblers with 

 whose eggs they agree very well. lu the Deccan, however, 

 the most common Warbler is Prinia socialis, the Ashy 

 Wren-Warbler, and this has accordingly been selected by 

 the Cuckoo as the most suitable nurse for its young. But 

 this Warbler lays red eggs, with which the great majority 

 of Cuckoos' eggs would contrast most conspicuously; in 

 consequence, therefore, a red egg has also been gradually 

 evolved for the Cuckoo by the foster-parents in each 

 generation destroying those which were most unlike their 

 own, until a red stage has been arrived at sufficient to 

 deceive the Warblers and ensure the perpetuation of the 

 Cuckoos. 



