LIMITED DEVELOPMENT OF CERTAIN INSTINCTS 98 



it is qiiite unnecessary to seek an explanation in consciousness, 

 and we have seen that they are due solely to the action of natural 

 selection acting on certain nerve centres. But, apart from these 

 cases, there axe others which perhaps show more clearly the rcie 

 of natural selection and the absence of conscious volition. 



Such are those interesting cases in which the instinct, already 

 present in a general form, is not specialised beyond the strict 

 limit of A^ital necessity to the species, beyond which selection has 

 no motive for favouring its development, although we might 

 consider such a development to be, if not necessary, at least 

 advantageous. There is a species of beetle, Sitaris humercdes, 

 belonging to the family of Cantharidee, whose larvae show appa- 

 rently no immediate instinct for taking food, but which fasten 

 upon a certain bee, whose egg — ]i the bee be a female — they sub- 

 sequently eat. If the bee, however, be a male, or if the larvsB 

 fasten upon another insect which is not a bee at all. the larvfe 

 must in either case perish for lack of sustenance. Xow, if the 

 instinct of the species were more developed, so that, instead of 

 its nerve centres being excited by the approach of any bee, they 

 were only excited by the approach of the female bees, it is certain 

 that a much greater number of larvae would survive and become 

 full-grown beetles than is the case at present. We must suppose 

 that the present degree of development of the instinct is, never- 

 theless, sufficient for the maintenance of the species, and that 

 there is no reason why natural selection should favour greater 

 multiplication of individuals. This case shows us, however, that 

 we have not to deal with conscious voUtion. For if conscious 

 volition had dictated the process of fastening on to the bee, there 

 is no reason why the instinct should not eventually have become 

 specialised to the degree of causing the nerve centres to enter into 

 activity ordy at the approach of a female bee.^ Obviously, the 

 aid of the Lamarckian doctrine of use and disuse might be 



1 Not to mention the fact that conscious volition is excluded as a 

 hypothesis in the case of a larva. 



