204 INSTINCT. CiiAr. VII. 



during several hours. After this interval, I felt sure that the 

 aphides would Avant to excrete. I watched thcni for some 

 time through a lens, but not one excreted ; I then tickled and 

 stroked them with a hair in the same manner, as well as I 

 could, as tlie ants do with their antennre; but not one excreted. 

 Afterward I allowed an ant to visit them, and it immediately 

 seemed, by its eager way of running about, to be well aware 

 what a rich Hock it had discovered ; it then began to play witli 

 its anteun;i3 on the abdomen first of one aphis and then of an- 

 other; and each, as soon as it felt the antenna?, immediately 

 lifted up its abdomen and excreted a limpid drop of sweet 

 juice, which was eagerly devoured by the ant. Even the 

 quite young aphides behaved in this manner, showing that the 

 action Avas instinctive, and not the result of experience. It is 

 certain, from the observations of Huber, that the aphides show 

 no dislike to the ants : if the latter be not present they are at 

 last compelled to eject their excretion. But as the excretion 

 is extremely viscid, it is no doubt a convenience to the aphides 

 to have it removed ; tlierefore, probably they do not excrete 

 solely for the good of the ants. Although there is no evidence 

 that any animal performs an action for the exclusive good of 

 another species, yet each tries to take advantage of the in- 

 stincts of others, as each takes advantage of the weaker bodily 

 structure of other species. So, again, 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, arc indispensable 

 for the action of natural selection, as many instances as pos- 

 sible 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 

 Avliolly 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. A\''hy, it has been asked, 

 if instinct be variable, has it not granted to the bee " the ability 

 to use some other material Avhen wax was deficient ? " But 

 what other material could bees use? They Avill work with 

 and use, as I have seen, wax hardened with vermilion and 



