276 MENTAL EVOLUTION IN ANIMALS. 



habit may, at an earlier period in the history of the species 

 or of its allies, have been of real utility. We may, for 

 example, readily imagine that the instinct displayed by many 

 herbivorous animals of goring sick or wounded companions, 

 is really of use in countries where the presence of weak 

 members in a herd is a source of danger to the herd from the 

 prevalence of wild beasts ; and Mr. Darwin in the Appendix 

 gives evidence that such is actually the case. Or, to take a 

 more fanciful illustration, we may suppose the Megapodidae 

 mentioned in the Appendix, which incubate their eggs by 

 placing them in a large heap of fermenting vegetable matter 

 which they collect for this purpose, were to find, from a 

 change of their habitat or of the Australian climate, that it 

 was difficult to collect a sufficient quantity of vegetable 

 matter, or that it would not ferment sufficiently for the pur- 

 pose of incubation. The birds might then gradually revert 

 to the usual mode of incubation, but might still retain a 

 marked propensity to make tumuli of vegetable matter as 

 nests. If so, the labour expended in making such tumuli 

 would be obviously useless, and there being no analogy 

 among the incubating habits of other birds to give us a clue 

 as to the origin of such an instinct, we should be quite at a 

 loss to account for it. 



Instincts apparently Detrimental to the Species which 

 exhibit them. 



It constitutes no difficulty or objection to our general 

 theory of instinct-formation to point to cases in which in- 

 stincts are obviously detrimental to the individuals which 

 manifest them ; for it is of the essence of the theory of 

 natural selection to suppose that the interests of the indi- 

 vidual are, in the process of selection, subordinated to those 

 of the species. It is, for example, manifestly to the detri- 

 ment of an individual fly to procreate its kind, inasmuch as its 

 own death is speedily induced by the act; but seeing that the 

 act is essential to the continuance of the species, we perceive 

 how natural selection must here have developed an instinct 

 which virtually amounts to that of suicide. And the same 

 remark appHes to all similar cases, such as that alluded to in 

 "Animal Intelligence" of soldier ants and termites sacrificing 

 themselves for the benefit of the community — i.e., the species. 



