190 ON THE ORIGIN OF SPECIES 



devoured by the ant. Even the quite young aphides 

 behaved in this manner, showing that the action was 

 instinctive, and not the result of experience. But as 

 the excretion is extremely viscid, it is probably a 

 convenience to the aphides to have it removed ; and 

 therefore probably the aphides do not instinctively 

 excrete for the sole good of the ants. Although I do 

 not believe that any animal in the world performs an 

 action for the exclusive good of another of a distinct 

 species, yet each species tries to take advantage of the 

 instincts of others, as each takes advantage of the 

 weaker bodily structure of others. So again, in some 

 few cases, certain instincts cannot be considered as 

 absolutely perfect ; but as details on this and other such 

 points are not indispensable, they may be here passed 

 over. 



As some degree of variation in instincts under a state 

 of nature, and the inheritance of such variations, are 

 indispensable for the action of natural selection, as 

 many instances as possible ought to be here given ; but 

 want of space prevents me. I can only assert, that 

 instincts certainly do vary — for instance, the migratory 

 instinct, both in extent and direction, and in its total 

 loss. So it is with the nests of birds, which vary partly 

 in dependence on the situations chosen, and on the 

 nature and temperature of the country inhabited, but 

 often from causes wholly unknown to us : Audubon has 

 given several remarkable cases of differences in the 

 nests of the same species in the northern and southern 

 United States. Fear of any particular enemy is 

 certainly an instinctive quality, as may be seen in 

 nestling birds, though it is strengthened by experience, 

 and by the sight of fear of the same enemy in other 

 animals. But fear of man is slowly acquired, as I 

 have elsewhere shown, by various animals inhabiting 

 desert islands ; and we may see an instance of this, 

 even in England, in the greater wildness of all our 

 large birds than of our small birds ; for the large birds 

 have been most persecuted by man. We may safely 

 attribute the greater wildness of our large birds to this 



