154 Darwin and Romanes dealt with. 



of some kind must operate to maintain so great a 

 disproportion. In our idea it is the large numbers of 

 birds — probably an increasing number — that turn out, 

 build over or destroy the cuckoo's egg, and refuse 

 to hatch it. By this the cuckoos are kept in check, 

 otherwise they would soon dominate other birds al- 

 together and decimate them. We see the same 

 process in operation, in many ways, by which the 

 balance of Nature is approachably maintained. This 

 is another and a new light on Mr. Romanes' random 

 and unfounded assertion that the deposition of a 

 cuckoo's egg must be " comparatively such an ex- 

 ceedingly rare event " that it was not worth while for 

 Nature to develop counteracting instincts. 



Unless by one or other agency of this sort, it is 

 evident that we should find some more definite relic 

 of these eggs that come to nothing."' 



XX. 



Another point of vast importance, which certainly 

 neither Mr. Darwin nor Mr. Romanes in the least 

 faced, is this, that wherever you find a disproportion 



* Gilbert White, in his letters to Daines Barrington, had 

 already questioned the statement that the cuckoo lays only one 

 egg and proposed to examine the ovarium so as to settle the 

 matter. 



Jenner found precisely what Gilbert White had expected — 

 that the ovary of the cuckoo was exactly like a hen's ovary, 

 with eggs in all stages, and he concluded, as White said he 

 would do, if the fact were so, that the cuckoo laid a great 

 number in each year. — Creighton's Jenner and Vaccination, pp. 

 12 and 13. 



