246 THE ZOOLOGIST. 
to twenty eggs; otherwise this species is strictly parasitic (see 
Darwin in his chapter on “ Instinct”’ in the ‘ Origin’). 
Of course, a large number of birds of the present day show 
a tendency to the primitive habit of leaving their eggs to hatch 
of themselves, viz. Emus, Grebes, Plovers, and others. Some 
cover their eges when leaving them (Anas boschas, and others) ; 
others, again, only during the period before the whole clutch is 
laid, while the bird is not sitting, viz. Parus major, P. ceruleus. 
And so, the male jealously guarding the eggs, or, in some cases, 
the female carefully secreting them, would ensure the safe pro- 
creation of the species, and that species would ultimately win in 
the struggle for existence. 
Though no one denies that Nature is profoundly extravagant, 
it is no contradiction of terms, and even more true to say that 
she is economical and thrifty too. No more eggs are laid by 
any bird than are necessary. With things at this juncture the 
necessity for so large a number of eggs would go, as the 
guarding male would eliminate so many of the risks. The 
reduction of eggs means an economy of food-yelk manufactured 
by the female, who would in turn receive an accentuation of 
vigour in some system other than her reproductive one. We 
must remember that Darwin’s great principle of the strugele 
for existence is universal, and occurs even between the integrate 
parts of the same organism. In this case the nervous system 
benefits at the expense of the reproductive, while the young 
birds would undoubtedly be stronger and healthier. The pro- 
duction of healthier chicks would probably be the cause of a 
second lessening in the number of eggs, and this would again 
react on the young, and so on ad infinitum. Furthermore, the 
parents, instead of attempting to cope with an unwieldy number, — 
would be able to give their undivided attention to the few chicks 
produced. 
But there would be limits. Every species ought to send out 
into the world annually enough young, not only barely to carry 
that species through along its own narrow groove, which its 
heredity and environment have cut for it, but also enough young 
to allow of the expansion of its area of distribution and power to 
cope with change of pressure ; for, under the rigorous action of 
