94 HEREDITY AND SELECTION IN SOCIOLOGY 



utilised in this case to explain " this specialisation by constant 

 use." On the other hand, the partial and incomplete nature of 

 the instinct is easily explicable if we suppose selection to have 

 originated it. Just as a machine can only work with the material 

 for which it is constructed, so an instinct can only induce an 

 appropriately adapted action when the animal is living under 

 natural conditions. The specialisation of any given instinct has 

 its limits; and this limitation implies the necessarily limited 

 utility of such speciaUsation for the species. We may say, how- 

 ever, that, in so far as the interest of the species requires special- 

 isation of any given instinct, natural selection will secure that 

 specialisation. The evolution of an instinct, whether its origin 

 be conscious and voluntary or not, is always the work of selection, 

 and is, consequently, unconscious and independent of the will. 



The last case of instinct which we shall notice is that exhibited 

 by the sea-cucumber, Cucumaria, in taking food. The tentacles, 

 ten in number, each with numerous branches, are well adapted 

 for catching small animals ; and numerous larvae of difierent 

 kinds, besides Infusorians and the like, come and settle on what 

 looks like a bush of seaweed. The cucumber, however, moves its 

 tentacles continuously up and down ; it brings the tip to its 

 mouth, slides the tentacle slowly down the gullet, and then with- 

 draws it, having " cleaned off " all the little living creatures 

 which are forthwith digested. This mechanical movement is 

 one of the few signs of life which the sea-cucumber exhibits ; and 

 the animal, being quite without nerve centres, cannot be credited 

 with volition, or even with consciousness. Its feeding instinct 

 is entirely automatic. In the same way, certain deep-sea fishes 

 have their eyes, not at the side of the head, but on the top ; and 

 the jaws are so constructed that they open upwards instead of 

 downwards. These fishes have an instinct for catching prey in 

 correlation with this morphological peculiarity. They bury them- 

 selves in the sand, so that nothing remains visible except their 

 eyes ; in this position they await the approach of a victim. 



