70 MIMICRY, 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 clanger, 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- 



