142 Mr. C. F. M. Swynnerton on Rejections [Ibis, 



colour to tlieir own " ; and I was interested lately, in looking 

 up the 'Origin' for his views on the subject of Cuckoos, 

 to see that the explanation I offered in ' The Ibis ' (Oct. 

 1916, p. 561) for the young Cuckoo's habit of ejection was, 

 in detail, that long ago given by Darwin. I am a bad 

 reader, having little time for it — I have probably not read 

 the ' Origin ' through since I was a schoolboy, \i then — and 

 the idea came to me independently, as it was likely to do to 

 anyone watching ejection and puzzled over its explanation. 

 I mention the point here merely in order to apologize for 

 having inadvertently brought forward the suggestion as 

 my own. 



I was interested further, however, to see that the general 

 theory of the transition to parasitism, as I have seen it 

 given by Newton and others, is also Darwin's. He refers 

 to the fact that various birds occasionally lay their eggs in 

 other birds' nests, quotes the Gallinaceae rather particularly 

 in this connection, refers to '' the singular instinct of the 

 Ostrich/' in which family *' several hen-birds unite and lay 

 just a few eggs in one nest and then in another, .... as with 

 the Cuckoo, at intervals of two or three days," and refers to 

 the fact that " the instinct of the American Ostrich, as in the 

 case of Mulothrus bonariensis, has not as yet been perfected, 

 for a surprising number of eggs lie strewed over the plains, 

 so that in one day's hunting I picked up no less than twenty 

 lost and wasted eggs." Darwin also speaks of the stages in 

 the transition that are illustrated by the American Cow- 

 birds, quoting from Hudson, and especially remarks on the 

 fact that in M. bonariensis, with parasitic habits already well 

 developed, " several [birds] together sometimes commence 

 to build an irregular untidy nest of their own," which they 

 apparently never finish, and that " they often lay so many 

 eggs — from fifteen to twenty — in the same foster-nest, that 

 few, if any, can possibly be hatched." 



It is possible out of Darwin's material, and with one or 

 two small additional suggestions, to frame the following 

 theory. So far as one can tell at this date, it may represent 

 an approximation to what has actually taken place. 



