marck, Lyell, and Darwin are the four greatest scientists who devoted themselves to this research) ends by being proclaimed by a philosopher as, so to speak, a logical necessity.

As the present short outline of the historical development of the idea of evolution shows, this idea has been established much more by scientific research than by philosophic speculation. And while the inorganic evolution was recognized by some of the Greek philosophers and by Descartes and Kant, the idea of organic evolution was never an integral part of any of the great systems of philosophy, and no great philosopher before Darwin had recognized the general and universal bearing of the idea of evolution. It is a rather extraordinary fact that an idea of an eminently philosophic importance, such as the idea of universal evolution, should never have been recognized as such until after science had demonstrated itthis is a bit humiliating to philosophers; and it shows us that human knowledge, if it wishes to attain its total unity, must find its support and its inspiration as much in scientific research as in pure speculation.

THE HEREDITY OF ACQUIRED CHARACTERS.1

By L. CUENOT,

Professor of the College of Sciences at Nancy.

An acquired character is a modification appearing in a plant or an animal, at any age whatsoever, which is plainly the effect of an exterior and accidental cause, of such nature that if this cause had not intervened, the modification would certainly not have been produced. Acquired characters are legion. We will cite a few to fix our conception of them and to illustrate the definition: The immunity which follows an infectious disease; the sensibility to injections of equine serum which is exhibited by the Tartar peoples nourished by the milk and meat of the horse; the pigmentation of the bare parts of the human skin exposed to the action of the open air and especially of light rich in ultra-violet rays; an accidental mutilation; the modifications presented by plants of the plain when transplanted to Alpine regions; the enlargement and the strength of a muscle systematically exercised; everything which man learns during his lifetime, such as his language, his writing, any form of sport, etc. It is understood that an acquired character is just the difference between the normal condition, or the condition which served as the point of departure before the action of the modifying cause, and the new condition after the action of that cause.

In order to consider a character as acquired, it is necessary that the relation of cause to effect should be evident, either when the cause has been made to act experimentally, or when the observation of nature has been made with the care and exactness of an experiment (which is rare). It is well known that for a long time, until about 1883, it was believed without question that the acquired character was hereditary to a more or less marked degree; that is, that parents having acquired a certain thing would procreate a generation presenting more or less completely, at least in the shape of an indication or a rudiment, the acquired bodily modification, the absence of the exterior cause which had produced it in the parents. Lamarck and his school made of this heredity of acquired

1 Translated by permission from the Revue Générale des Sciences, Oct. 15, 1921.

in

characters the pivotal point of their theory of evolution and of adaptation. Darwin and Herbert Spencer also fully accepted it. It is useless to insist on the capital importance of the phenomenon for the general explanation of adaptations. Without for the moment asking ourselves why, we can see that the reactions of the individual to any exterior action whatever have very generally an adaptive or protective value: A tanned skin is less penetrable by ultraviolet rays; immunity protects against a new attack of disease; a muscle, a joint, exercised within certain limits, functions more easily and more effectively than before the training. If the heredity of acquired characters exists, to however slight a degree, we possess the key to an enormous number of adaptations. If it does not exist, we must find other explanations.

This is not alone a question which interests speculative scholars isolated from the world; it is also a question of importance to society. When it is repeated to the public at large that the practice of sports, even to excess, prepares for vigorous new generations, the idea is certainly entertained that the "all-round athletes" or even those who are abnormally specialized by exercise, will bequeath to their descendants at least a rudiment of their acquired qualities. It is surely the opinion of breeders, who believe that the effects of the training of race horses, of the good or bad nutrition of cattle, are transmissible in certain measure.

It was not until 1883 that, for theoretical reasons whose value has not been diminished by time, Pflüger, on the one hand, and Weismann, on the other (Essay on Heredity, read at a public meeting when he was tendered the position of vice rector of the University of Freiburg, on June 21, 1883), were led to formally doubt this heredity. Weismann presented arguments of such force-he examined the whole question with so penetrating a critique—that it is only just to give him the credit for the change of opinion which dates from his lecture of 1883. But if he convinced many biologists, he encountered also unyielding opponents so powerful that for 37 years, in spite of numerous and remarkable researches, the heredity of acquired characters has remained a problem continually presented. It may be said that all the experimental proofs which have been contributed to the support of the transmission (E. Fischer, Standfuss, Kammerer, etc.) are mediocre and do not lead to conviction, or are susceptible of criticism and interpretations which weaken their demonstrative value, or, indeed, are frankly contradicted (as in the

2 Pflüger, Ueber den Einfluss der Schwerkraft auf die Theilung der Zellen und auf die Entwicklung des Embryo, Arch. f. Phys., t. XXXII, 1883, p. 68.

Weismann, Essais sur l'Hérédité et la sélection naturelle, trad. de Varigny, Paris,

case of the mutilations and the experiments of Brown-Séquard). In truth the decisive experiment, verified and certain, is still lacking. As for the nonexperimental proofs, they are always subject to discussion and can never attain beyond second rank.

As it is necessary, however, to hold some opinion on a question so important that it dominates all the conceptions of evolution, we must decide for ourselves according to the relative weight which we attribute to the various arguments, and naturally each one of us holds the more strongly to his own view, the more this view allows for a greater degree of personal appreciation. Some, partisans of heredity, receive with pleasure experiments or observations, even mediocre, which seem to them to constitute proofs; others who reject it, hunt for weak points in the demonstrations, and usually find them. It is almost a matter of faith, of nationality. The great majority of French zoologists favor the affirmative, following Giard, Edmond Perrier, Le Dantec, F. Houssay, Delage, all more or less Lamarckists; the Americans, except some paleontologists, are nearly all for the negative. I am perfectly ready to admit that up to the present I have put myself on the negative side.

It is obvious that there are categories of acquired characters arising from different causes: (1) Mutilations; (2) the effects of parasitic diseases producing a general intoxication; (3) the action of the great natural factors, light, temperature, humidity, salinity, nourishment; (4) the effects of use or those of disuse; (5) the psychic acquisitions of training, of instruction. In my opinion, the negative demonstration, that of the nonheredity of acquired characters of one of these categories, is of value for that one only, and can not legitimately be extended to the others, for if there are certain acquisitions of the body which are not transmitted to the sexual cells, the bearers of the hereditary patrimony, it does not necessarily follow that the same is true for all. But, on the other hand, if there were an experiment which showed indisputably the heredity of a truly acquired character, it would be a strong probability for some other categories, for although we do not understand completely how an acquired bodily modification can add itself even in weakened form to the hereditary patrimony, if the fact were proved one single time, the argument of incomprehension would lose all of its force.

For the first category of acquired characters, it can be said that the answer is definite; since the critiques and the experiments of Weismann, many times repeated, no one believes any longer in the heredity of mutilations. Every-day observations confirm those of biologists and it is certain that the pseudo-examples of the transmission of mutilations that are quite often reported among domestic

animals and among human families are simple coincidences, which have no more interest and often no more authenticity than cases of supposed maternal impressions. The experiments of Brown-Sequard, badly done moreover, which concern the heredity of mutilations and that of physiological disorders following nervous mutilations, have been completely disproved by researches which inspire confidence, and there is nothing left of them.

Various authors have affirmed the heredity of acquired characters of the fifth category, as little likely as this seems. Here are some examples which Hachet-Souplet reports as demonstrative. A macaque monkey which he had taught, not without difficulty, to kill rats, gave birth to young who hunted rats marvelously; cats trained to respect mice had young which did not take mice, even when the distribution of their food was intentionally retarded; sparrows trained to draw a chain from a little well for six generations gave birth to young which, without training, were able to draw the same chain. A dog had been trained to make rapid pirouettes to the left; a daughter of this dog, raised in the country, having no example before her eyes and having received no training, began by herself to make pirouettes to the left toward five or six months of age. All that is very astonishing. Although one can not criticize experiments which he has not followed, I am persuaded that there is a "hole" in these observations, due perhaps to the deceit of assistants, to a surreptitious training continued unobserved, etc., and I do not doubt that the heredity of acquisitions of training will go to join that of mutilations.

The third category concerns the factors of the medium; no one doubts their determining influence on the characters of animals and plants, and it is certain that when they have been made to vary experimentally up to the extreme limits compatible with life there often result notable modifications among beings which are submitted from youth to a change of environment. But the question is to know whether these modifications pass, even in a very attenuated degree, to the succeeding generation reared in the normal medium. If they do, we have the key to the formation of geographical races and of many adaptations; for it is a fact that the results of the action of the medium would necessarily be cumulative, and after a sufficient number of generations passed in the modifying medium, the species might be very notably transformed, perhaps even irreversibly; if they do not, the effects of the medium would be produced anew for each individual, without cumulation, and the influence of the environment on the body would no longer have any interest from the point

Hachet-Souplet, La genèse des instincts, étude expérimentale, Bibliothèque de Philos. scient., Flammarion, Paris, 1912. P. 239.