70 MIMIORY, AND OTHER PROTECTIVE 
The tortoise is not injured by the conspicuous colours 
of his shell, because that shell is in most cases an effec- 
tual protection to him. The skunks of North America 
find safety in their power of emitting an unbearably 
offensive odour; the beaver in its aquatic habits and 
solidly constructed abode. In some cases the chief 
danger to an animal occurs at one particular period of 
its existence, and if that is guarded against its numbers 
can easily be maintained. This is the case with many 
birds, the eggs and young of which are especially ob- 
noxious to danger, and we find accordingly a, variety 
of curious contrivances to protect them. We have 
nests carefully concealed, hung from the slender extre- 
mities of grass or boughs over water, or placed in the 
hollow of a tree with a very small opening. When 
these precautions are successful, so many more indivi- 
duals will be reared than can possibly find food during 
the least favourable seasons, that there will always be a 
number of weakly and inexperienced young birds who 
will fall a prey to the enemies of the race, and thus 
render necessary for the stronger and healthier indi- 
viduals no other safeguard than their strength and 
activity. The instincts most favourable to the produc- 
tion and rearing of offspring will in these cases be 
most important, and the survival of the fittest will act 
so as to keep up and advance those instincts, while 
other causes which tend to modify colour and marking 
may continue their action almost unchecked. 
It is perhaps in insects that we may best study the 
varied means by which animals are defended or con- 
