THEORIES AND CONCLUSIONS 231 
rapidity in carrying out the actual parasitic function 
has become a condition of success. 
It may have puzzled many observers, as it 
always puzzled me, to find a reason why the Cuckoo 
lays an egg so small in proportion to her own size. 
Here, again, the proved fact that she frequentl} 
has to await a favourable opportunity for the 
actual deposit of her egg supplies the answer, for 
it is clear that the smaller the egg compared to the 
bird's size, the more easily can she retain it even 
when it is ready to be laid. 
Another facet of the Cuckoo problem that 
seems to me to be illuminated by the law of the 
survival of the fittest, necessitates a brief con- 
sideration of the similarity between eggs. In 
my own collection of hundreds of Cuckoos' eggs, 
taken from the nests of several species of foster- 
bird, I can show Hedge-sparrow Cuckoo’s eggs 
with a tendency to bluish-green in the ground 
colour and faint surface-markings ; Meadow Pipit 
Cuckoo's eggs with all the superficial characteristics 
of Meadow Pipit's eggs ; Wagtail Cuckoo's eggs 
with that generally grey appearance typical of 
Wagtail's eggs ; and Robin Cuckoo's eggs exhibiting 
a reddish tinge. I am confident that other practical 
observers and collectors will agree that this is 
