From the preceding facts we can conclude, contrary to the disciples of Weismann or the neo-Darwinians, that instincts may be modified by the hereditary transmission of slowly acquired habits, and, contrary to the pure neo-Lamarckians, that new instincts may appear suddenly by mutation. Darwin arrived at the same conclusions. "It would be one of the gravest errors, ' ' he wrote in his treatise on ' ' The Origin of Species," "to believe that most instincts have been acquired by habit in a generation and transmitted by heredity to following generations." And he added, "I believe that the effects of habit are of a quite secondary importance if we compare them to natural selection upon what we can call accidental variations of instinct,-that is to say, variations produced by the same unknown causes which bring about slight variations in the structure of the body." These accidental variations are nothing but mutations. Darwin believed them always very feeble, which seems to be contradicted in the case of Ortmannia, and he thinks that they are accentuated by the action of natural selection. He believes, also, that they play a greater part in the evolution of instincts than the inheritance of habits, but that is a point which it is impossible to confirm in our present state of knowledge.
However it may be, the two schools agree in attributing the evolution of instincts to heredity,- that is, to modifications in the intimate nature of the germ cell, brought about by different influences, which can maintain themselves as long as other influences do not modify them in turn.
These intimate modifications of the germinative cells do not always manifest themselves in the structure or the instincts of the descendants. Some of them remain latent for a longer or shorter time; then suddenly their presence is noted with certain individuals having the ancestral characters. This is the phenomenon to which the name atavism has been given. It is frequently noted with man, where it is seen especially when the children resemble their parents much less than their direct or collateral ancestors.
The experiments of Schroeder (1908) upon the larvae of a small moth, Grciciliana stigmatella, furnish a beautiful example of atavism and of rapidity in the acquiring of habits. These caterpillars have the habit of enclosing themselves in a case which they make from the end of the pointed leaves of a species of willow (Salix alba, variety vitellina) and line with silken threads. They do the same on the black poplar, the leaves of which are drawn out into a point. But their instinct does not lack flexibility. Schroeder observed that, taken out of the case, which has used up much silk, they can make another, or simply roll up the edge of a leaf or even draw together by threads two or three contiguous leaves, and he notes that in doing this sort of thing they show instincts which are found with other Microlepidop-tera whose work is more primitive. Analogous variations are again observed, although of a greater intensity, when one places a caterpillar with leaves of which it is impossible to make a typical case. Thus with the rather obtuse leaves of Salix aurita there is a simple fold of the edge just as with the leaves of Salix viminalis when the tips have been cut off. After placing ninety-four caterpillars on such leaves Schroeder found that eighty-four formed a shelter by folding the edges, the others trying with varying success to roll the cut ends of the leaves into cones. The second generation, to the number of forty-three, all appeared to have acquired the instinct of folding the edges of the leaves. But nineteen caterpillars of the third generation having been placed upon normal leaves, fifteen of them made cones and four continued to fold the leaf edges. Schroeder considered this a double example of atavism : the first fifteen had undoubtedly gone back to the instinct of their grandparents, but the four others had shown rejuvenated primitive habits of ancestors much farther back. Let us say in passing, with Schroeder, that it is very difficult to explain these facts as variations in the germ cells, that there is a reciprocity in the relations of the germ plasm to the soma, and that, according to circumstances, the variations of both can be hereditarily efficacious.
Atavistic manifestations show the variability of instincts and the hereditary power as to the variations of instincts, but, from the fact that they revive ancient habits often acquired a long time past, they cannot be confused with actual variations. Wheeler (1905) has reviewed many phenomena which he considers related to atavism, notably the faculty which the domestic honey-bee has of sometimes nesting in the open air, and the Polistes of temperate regions of storing honey like the wasps of the same genus which live in the tropics. In these two cases atavism does not seem doubtful, but examples may be cited where the embarrassment is greater. Ammophila hirsula, variety mervensis, is a predatory wasp which chooses for its victims cutworms of the genus Argotis and which has an especial agility in relation to the quick movements and struggles of its prey. As Ferton has noted, after Fabre, this wasp has a firmly fixed instinct. But Ferton has seen the same species capture a large, above-ground caterpillar, Cucullia chamomillce, and in another case the hairy caterpillar of Epinephele jurtina which lives on grasses, and we know very well that the Ammophilas avoid hairy caterpillars. Ferton says :
In these two cases, has the Ammophila, not being able to find the cutworm which she is looking for, abandoned her usual hunting-method to develop another ? I do not think so, and to admit it would be to give the creature too much intelligence. ... In my opinion it is atavism which has led my two specimens to paralyze these other caterpillars. The instinct of A. hirsuta is not yet sufficiently fixed, and it happens that the female in hunting found herself near a larva which was not her normal prey, but nevertheless stung it and paralyzed it in order to give it to her larvae, just as her ancestors had done.
This explanation is a convenient one, but without any proof, like most of the explanations founded on atavism. It is easy to invoke the past, but you cannot always have a response; or, rather, as Emery has shown (1902), we reply subjectively by means of comparisons which allow us to confound simple homologies with real atavistic phenomena. However this may be, this story of Ammophila hirsuta is of interest because it shows that this wasp is capable of profoundly modifying its predatory habits when necessary.