458 The Bird 
hatched and reared by foster-parents. Such an unusual, 
almost unique habit has brought about a considerable 
modification of the eggs. Anything which would tend to 
deceive the greatest number of intended victims would, 
of course, greatly redound to the advantage of parasitical 
birds. 
The remarkable similarity of the English Cuckoo’s egg 
to those in the nest in which it is laid has been explained 
as due to each individual bird being accustomed to lay its 
ege in the nest of the same species favoured by its parents 
and its more distant ancestors; its eggs in course of time, by 
natural selection, thus coming to resemble the eggs of that 
particular species. Other adaptations are the extremely 
small size of the egg in comparison with the parent bird, 
and also the unusual strength and weight of the shell. 
This last is doubtless of great value; for, strange as it 
may seem, the bird first deposits its egg upon the ground 
and then picks it up in its beak and places it in the nest 
selected. Thus a strong shell is a very necessary require- 
ment. 
The colours of eggs have been carefully examined with 
the spectroscope and are found to consist, chemically, of 
seven pigments: a brownish red, two delicate blues, two 
clear yellows, a peculiar brown hue, while the seventh is a 
rather indefinite shade, known as lichenixanthine—most 
interesting of all as being identical with a colour substance 
common in plants and especially in lichens and fungi. 
These substances somewhat resemble those found in the 
blood and the bile. They are deposited on the shell while 
the egg is passing down the oviduct, and it is to the circular 
