The experiments which we have just described are almost all very recent, and they will have to be added to somewhat, but they establish the fact that insects acquire new habits very quickly and that these acquired habits become hereditary and that they take the form of instincts. "For a long time," wrote Lamarck,1 "we have had in this respect a feeling of what is the case, since it has come to be a proverb that Habits become second nature."
I do not believe, as a matter of fact, that we can literally apply to these phenomena the law of heredity of acquired characters according to the formula of the illustrious zoologist.
All that nature has made individuals or their race acquire or lose by the influence of circumstances has been long in achievement; for example, by the influence of the predominant employment of such an organ or by the constant lack of use of such a part. This is passed on by a generation to new individuals provided these acquired changes are common to the two sexes or to those which have produced these new individuals.
1 Lamarck, Philosophie zoologique, 1809.
This law appears to me to lend itself to two objections, the one relating to habits and the other a very general character.
The first is of slight importance, since to cause its disappearance it sufiices to modify very slightly the Lamarck text. As to habits at least, the hereditary transmission of acquired variations does not necessitate always a very long "influence of circumstances99 upon the race. This action has been exercised, no doubt, for a long time with the silkworm, but in the experiments of Schroeder and Pictet the inheritance was acquired at the end of a very few generations, and in that of Marchai on the scale insects it was complete with the second generation.
The second objection appears to me to be much more serious, and that is why I have italicized the clause. If it is true that one of the necessary conditions of heredity is the coexistence of the same acquired character in the two procreatore, how is it possible to explain with the Lamarckian theory the acquisition of morphological characters which distinguish the male from the female animal? and, to confine ourselves to the subject which we are discussing, how can the evolution of instincts in most insects be explained? With the latter, as a matter of fact, it is the female almost always which occupies herself with breeding, which hunts, collects, makes nests ; and it is consequently with the individuals of this sex that we observe the most interesting instinctive habits. With a good number of dung-beetles and certain other forms where both sexes work conjointly in the same labor, the male lives only for himself and often even perishes soon after coupling; and his instincts are infinitely less complex and quite different, for the most part, from those of the female. If, then, we accept the principle established by Lamarck, we must renounce the Lamarckian explanation of the very curious instincts which we observe with the sacred dung-beetle and with the solitary wasps, just as we cannot comprehend the adaptive modification of the organs which the females of these insects show. If it is true that, in order to be transmissible by heredity, the acquired changes must be common to both sexes, the Lamarckian law completely destroys the Lamarckian evolutionary theory. In his interesting work upon the "Crisis of Evolution," LeDantec insists at length upon this Lamarckian law and stresses the importance of his final restriction, but his opinion was certainly not shared by the lamented Giard, for, in his "General Principles of Biology," published in 1876, and his "Evolutionary Controversies," appearing in 1904, the eminent zoologist modifies the law of Lamarck and does not mention the restriction which I have italicized above. For the rest, if acquired characters are hereditary they should be governed by the Mendelian law, and this law specifically shows that two procreatore marked with different characters bear in the first generation individuals with mixed characters (hétérozygotes), in the second a certain number of similar individuals with others which are quite pure (homozygotes), some of the male character and others of the female character.
In fact, there has not been tried, down to the present time, any experiment to settle the question whether or not, with species having very different males and females, acquired characters are hereditary, and I have remarked how desirable it is that experiments should be made in this way with such forms as Ferton's pompilids, the Sphex flavipennis of Fabre, and the Ammophila urnaria of the Peckhams; but aside from what I have just said I believe that it will be necessary in the meantime to follow the example of Giard and remove the final restriction from the Lamarckian law. We will admit that acquired characters are hereditary, not only in species where the two sexes have the same habit, as is established by the experiments of Marchai, Schroeder, and Pictet, but also with those in which the two sexes are from this point of view profoundly different; in fact, we need only conform to the law of Mendel.
It is clear, moreover, that natural selection in the course of generations causes all the individuals to disappear which do not present the acquired character in a pure state, since this latter is really advantageous to the race. For example, the employment of a tool is advantageous in making the nest: the females of Ammobilila umaria which have acquired this habit, in pairing with males in whom naturally the habit cannot exist, will give female individuals of mixed characters, others with pure characters, some of the male type and others of the female type, in everything concerning habits.
Lamarck seems to have foreseen the objection just formulated, for in the commentary on his law he expressed himself in the following way : "In reproductive reunions, the mixtures among the individuals which have different qualities or forms are necessarily opposed to the constant propagation of these qualities and these forms. '• I have italicized the word constant because it shows that Lamarck did not ignore that in the progeny of two individuals with dissimilar characters certain individuals show the characters of the male and certain others of the female. But the illustrious zoologist had not foreseen natural selection, the efficacious role of which was put in evidence by Darwin, and therefore he could arrive only at the conclusion that the characters advantageous to one of the sexes would finish by belonging to all of the descendants in the course of successive generations. Indeed, Lamarck expressed a fact of experience when he affirmed that acquired characters, particularly of one of the sexes, are not fixed with all the descendants of a caterpillar; but it must be added to it that these characters, when they are advantageous, end in the long run by becoming constant, thanks to the law of natural selection. It was this, without doubt, which warned Giard when he discarded from the La-marckian law its final restriction.
How can we explain the hereditary transmission of acquired characters Ì In regard to the caterpillars raised by Pictet, Bohn (1913) observed that a change of the nourishment often results in a diminution of the activity of the organism. He says :
If it is a caterpillar, the adult which comes from it is smaller and paler and less energetic ; it has a lessened vitality, a lessened muscular energy,-that is to say, a lessened chemical activity. It is precisely this which it transmits to the next generation. The heredity of the new habit would be only an illusion. What it inherits would be simply an enfeebling of the chemical activity of the organism, and there is nothing mysterious about that. One inherits especially predispositions.
It is well also, as a matter of fact, that one should understand in these days the theory of Lamarck; but in the example cited by Bohn the heredity of the new habit is not an "illusion," since the chemical diathesis transmitted by heredity is the consequence of this habit which it will provoke anew in its descendants.
Is it necessary to believe with the same author that modifications of the chemical condition can also be produced by "causes foreign to the functioning," that is to say, independent of the Lamarckian explanation? That is not our opinion. For Lamarck the influence of the environment controls the predominant use or lack of use "of such organ" or "of such part," and by organ or part we should understand not only the external mechanisms of the animal but the tissues or groups of tissues which work and secrete in the interior of the body. We believe with Bohn that the atrophy of the wings of many insects is not the result of the non-use of these organs, and we do not ignore the obvious fact that the extraordinary ornaments of certain articulates (the mandibles of the stag-beetles, the frontal or thoracic horns of some scarabs, the spines of the Gasteracanth spider) are not the result of predominant usage. These characters come, without doubt, from chemical modifications and, without doubt also, from internal functionings which depend upon environment and circumstances.