THE INSTINCTS OF ANIMALS 151 



The little animals have at first no food-instinct, or at least none 

 manifests itself, but they run about, and as soon as they see a bee of 

 the genus Anthophora they spring upon it and hide themselves in its 

 thick, hairy coat. If they have been fortunate the bee is a female, 

 who founds a new colony and builds cells, in each of which she 

 deposits some honey and lays an egg upon it. As soon as this has 

 been done the Sitaris larva leaves its hiding-place, bites the egg of 

 the bee open, and gradually eats up the contents. Then it moults, 

 and takes the form of a grub with minute feet and imperfect masti- 

 cating organs ; the tail-fork, too, is lost, for all these parts are now 

 useless, since it can obtain liquid nourishment without further change 

 of place, from the honey in the cell, in exactly the quantity necessary 

 to its growth. Then it spends the winter in a hardened, pupa-like 

 skin, and it is not till the next year (the third), after another short 

 larval stage (d) and subsequent true pupahood (e), that the fully- 

 formed beetle emerges. This again possesses biting mouth-parts, and 

 eats leaves, and has legs to run with and wings to fly with. 



In this beetle, then, the food^instinct changes three times in the 

 course of its life ; first the egg of the bee is the liberating stimulus, 

 then the honey, and finally leaves. The instinct of moving about 

 varies likewise, expressing itself first in running and jumping and in 

 catching on, then in lying still within the cell, and, lastly, in flying 

 and running about on bushes and trees. 



We can well understand that, in the course of innumerable 

 generations and species of insects, the various stages of development 

 would, by means of selection, become more and more difierent from 

 each other, both structurally and in their instincts, as they adapted 

 themselves better to difierent conditions of life; and thus ultimately 

 instincts frequently and markedly divergent have been developed 

 in the successive stages of life. No other interpretation is possible ; 

 through natural selection alone can we understand even the 

 principle of such adaptations. An animal can thus very well be 

 compared to a machine which is so arranged that it works correctly 

 under all ordinary circumstances, that is to say, it performs all the 

 actions necessary to the preservation of the individual and of its 

 kind. The parts of the machine are fitted together ■ in the best 

 possible way, and work on each other so ingeniously that, under 

 normal circumstances, a result suited to the end is achieved. We 

 have seen how precisely the liberating stimulus for an action may be 

 defined, and this secures a far-reaching specialization of instincts. 

 But as every machine can work only with the material for which 

 it was constructed, so the instinct can only call forth an action 



