If instinct be, as Professor Hering, Mr. Samuel Butler, and others have argued, "hereditary habit"--i. e. if it comprises an element of transmitted experience--we at once find a complete explanation of many cases of the display of instinct which otherwise remain inexplicable. For although a large number--or even, as I believe, a large majority--of instincts are explicable by the theory of natural selection alone, or by supposing that they were gradually developed by the survival of fortuitous variations in the way of advantageous psychological peculiarities, this only applies to comparatively simple instincts, such as that of a protectively coloured animal exhibiting a preference for the surroundings which it resembles, or even adopting attitudes in imitation of objects which occur in such surroundings. But in all cases where instincts become complex and refined, we seem almost compelled to accept Darwin's view that their origin is to be sought in consciously intelligent adjustments on the part of ancestors. Thus, to give only one example, a species of Sphex preys upon caterpillars, which it stings in their nerve-centres for the purpose of paralyzing, without killing them. The victims, when thus rendered motionless, are then buried with the eggs of the Sphex, in order to serve as food for her larvae which subsequently develop from these eggs. Now, in order thus to paralyze a caterpillar, the Sphex has to sting it successively in nine minute and particular points along the ventral surface of the animal--and this the Sphex unerringly does, to the exclusion of all other points of the caterpillar's anatomy. Well, such being the facts--according to M. Fabre, who appears to have observed them carefully--it is conceivable enough, as Darwin supposed[48], that the ancestors of the Sphex, being like many other hymenopterous insects highly intelligent, should have observed that on stinging caterpillars in these particular spots a greater amount of effect was produced than could be produced by stinging them anywhere else; and, therefore, that they habitually stung the caterpillars in these places only, till, in course of time, this originally intelligent habit became by heredity instinctive. But now, on the other hand, if we exclude the possibility of this explanation, it appears to me incredible that such an instinct should ever have been evolved at all; for it appears to me incredible that natural selection, unaided by originally intelligent action, could ever have developed such an instinct out of merely fortuitous variations--there being, by hypothesis, nothing to _determine_ variations of an insect's mind in the direction of stinging caterpillars only in these nine intensely localized spots[49]. [48] For details of his explanation of this particular case, for which I particularly inquired, see _Mental Evolution in Animals_, pp. 301-2. [49] Note B.