Adaptation in Parasitic Cuckoos' J^i/ffs. 385 



point to the striking similarity between some Pipits' eggs and 

 the Cuckoos' eggs which are placed in their nests, or to the 

 many cases in Avhich the Cuckoos' eggs have been found in 

 the nests of Acrocephalus, requiring quite a careful examina- 

 tion before it can certainly be decided that they are not 

 aberrant ones of the Warblers themselves; or again, many 

 blue eggs of Cuculus canorus have been found in nests of the 

 lleclstart, differing only in size and texture from the eggs of 

 that bird. 



But there are still ornithologists who declare that no 

 such process of evohition is in existence, and that there is 

 practically no law which governs the Cuckoo's practice as 

 to where it places its eggs. These authorities, in order to 

 enforce their arguments, point to the Cuckoos' eggs which 

 are so often found in other birds' nests together with eggs 

 with which they have absolutely nothing in common ; and, if 

 one oologist, trying to prove them wrong, says " How is it 

 that so many blue eggs are found in Redstarts' nests ?" the 

 other side replies, " How is it that so many Cuckoos deposit 

 their eggs in the Hedge-Sparrows' nests, yet so very few are 

 found to be blue? " Both are right in what they say, only 

 it is hardly fair to lay down general laws after a study of but 

 one species, and, therefore, before drawing any deductions 

 many species should be studied. Also it should be under- 

 stood that in the case of the evolution of the Cuckoo's egg 

 nature is working much as she works in other cases. 

 Evolution here only means the elimination of the unfit ; 

 creation is the result of destruction, and the process of perfect 

 adaptation is attained by the slow but sure elimination by the 

 foster-parents of those eggs which contrast most distinctly 

 with their own. It is not the Cuckoo that evolves a perfectly 

 adapted egg but the foster-parents, which gradually eliminate 

 the types of Cuckoos' eggs that are so ill adapted as to 

 attract their notice. By this means those strains of Cuckoos 

 which lay the most ill-adapted eggs gradually die out, whilst 

 those that lay eggs most like those of the fosterer are 

 enabled to persist. 



Even this process is, however, checked or accelerated by 



