14G THE EVOLUTION THEORY 



thoracic' Icus is turiicMl towards the back, instead of being set at the 

 side of the body, as is \isual among crabs. They are thus enabled 

 to lu)ld the sponge much better and more permanently, and as this is 

 advantageous we may well ascribe the change to natural selection. 



Let us now turn our attention to another category of instincts, 

 the most common and most indispensable of all, those which lead to 

 the seekinu: and devouring of food. 



The chicken just emerged from the egg picks up the seeds thrown 

 to it with no experience of what eating is, or what can be made to 

 serve it as food ; its instinct for food expresses itself in picking up, 

 and it is awakened or stimulated to action by sight of the seeds. As 

 Lloyd Morgan in his excellent book on Habit and Instinct well says, 

 ' It does not pick at the seeds because instinct says to it that this 

 is something to be picked up and tested, but because it cannot do 

 anything else.' 



In the same way the instinct to seek for food wakes in the kitten 

 at the siirht of a mouse. I once set before a kitten which had never 

 seen a mouse a living one in a trap. The kitten became greatly 

 excited, and when I opened the trap and let the mouse run away she 

 overtook and cauo-ht it in a few bounds. The instinct in this case 

 does not express itself as in the chicken by the rapid lowering of the 

 head and seizing the food, but in quite a different combination of 

 movements, in running after and grasping the fleeing victim. But 

 that is not all that is included in the instinctive action in the case of 

 the cat, for there is also the whole wild and gruesome play, the 

 familiar letting go and catching again, the passionate growling of 

 satisfaction which, in its w^ildness, reminds us much more of a blood- 

 thirsty tiger than of a tame domestic animal. 



As the egg-laying instinct of the female butterfly is excited only 

 by the sight and odour of a particular plant, so also is the food 

 instinct of the caterpillar. If we put a silkworm caterpillar (Bombyx 

 mori) just out of the egg upon a mulberry leaf it will soon begin to 

 gnaw at it ; but put it on a beech leaf or on that of any other indi- 

 genous tree, shrub, or herb and it will not touch it, but simply die of 

 hunger. And yet it could quite well eat many of these leaves, and 

 thrive on them too, but the smell and perhaps also the sight of them 

 is not the appropriate stimulus to liberate the instinct of eating. 

 There are many species of caterpillar w^hich are ' monophagous,' that 

 is to say, restricted to a single species of plant in a country. One may 

 ask how such a restriction of the liberatino- stimulus to a single 

 species could have been brought about l)y natural selection, since it 

 could not possibly be advantageous to be so much restricted in food. 



