294 ‘The Naturalist in Nicaragua 
blances, but to increase the perceptions of the predatory 
species of insects and birds, so that there is a continual pro- 
gression towards a perfectly mimetic form. This progressive 
improvement in means of defence and of attack may be illus- 
trated in this way. Suppose a number of not very swift 
hares and a number of slow-running dogs were placed on an 
island where there was plenty of food for the hares but none 
for the dogs, except the hares they could catch; the slowest 
of the hares would be first killed, and the swifter preserved. 
Then the slowest-running dogs would suffer, and having less 
food than the fleeter ones, would have least chance of living, 
and the swiftest dogs would be preserved; thus the fleetness 
of both dogs and hares would be gradually but surely per- 
fected by natural selection, until the greatest speed was 
reached that it was possible for them to attain. I have in 
this supposed example confined myself to the question of 
speed alone, but in reality other means of pursuit and of 
escape would come into play and be improved. The. dogs 
might increase in cunning, or combine together to work in 
couples or in packs by the same selective process; and the 
hares on their part might acquire means of concealment or 
stratagem to elude their enemies; but, on both sides, the 
improvement would be progressive until the highest form of 
excellence was reached. Viewed in this light, the wonderful 
perfection of mimetic forms is a natural consequence of the 
selection of the individuals that, on the one side, were more 
and more mimetic, and on the other (that of their enemies) 
more and more able to penetrate through the assumed dis- 
guises. It has doubtless happened in some cases that species, 
having many foes, have entirely thrown off some of them 
through the disguises they have been brought to assume, but 
others they still cannot elude. 
Since Mr. Bates first brought forward the theory of mimetic 
resemblances its importance has been more and more demon- 
strated, as it has been found how very largely animal life has 
been influenced in form and colour by the natural selection 
of the varieties that were preserved from their enemies, or 
enabled to approach their prey, through the resemblance 
they bore to something else. So general are these deceptive 
resemblances throughout nature, that it is often difficult to 
determine whether sexual preferences or the preservation of 
