The human species

by A. de Quatrefages

It is not the resemblances existing between the members of the same species, or between the members of one family, which perplex philosophers: all agree in referring them to heredity. The problem lies rather in the differences. Not only in the considerable differences which are established between races; but more especially in the shades constituting the individual traits which distinguish father from son, or brother from brother. This is in reality the fundamental difficulty, and many hypotheses have been proposed for its solution. Prosper Lucas, after having discussed them separately, regarded them all as insufficient, and believed that, side by side with heredity, which maintains types, we ought to admit a special force, innateness (l’innéité) which diversifies them.

We can, however, account for the double tendency exhibited by living beings, without having recourse to a new force. For this purpose it is sufficient to push the analysis of phenomena a little further than is customary, and to obtain a clear idea of the part played by the conditions of life (milieu) and heredity. As a general rule an action is attributed to the first, which everywhere and at all times is a modifying one, and to the second a purely conservative action. Now it may be easily shown that this is not the case; and that each of these causes acts in an inverse manner according to circumstances.

IV. By virtue of the laws of heredity, the father and mother tend equally to transmit to their offspring their own character. However similar they may be supposed to be, there are always some differences between them; and the nature of the new being is necessarily a compromise between two different tendencies. The son cannot, therefore, always resemble his father exactly. In him the characters common to both parents will easily be exaggerated; the opposite characters will be neutralised; and the different characters will produce a resultant, as distinct from the two components as green is from yellow and blue. Thus even by virtue of its own tendencies, and in consequence of the enforced co-operation of the sexes, direct and immediate heredity becomes, in some respects, a cause of variation.

Mediate and indirect heredity, justly compared by Burdach to geneagenetic phenomena, as well as atavism, which suddenly reproduces with great exactness the characters of an ancestor, sometimes after hundreds of generations, have certainly considerable influence in the variation of individual traits, and in the differences which distinguish parents from their children.

Their action, added to that of direct heredity, is sufficient to explain the appearance of certain varieties, without appealing to innateness.

V. But the hereditary force, although it is manifested from one generation to another, or through several generations, is always influenced by the conditions of life (milieu), and this has evidently greater force.

This term ought to be taken in a much more general sense than is usually the case. Buffon himself only took into account climate, varying quantities of food, and the hardships of servitude, when he was treating of domestic animals. I understand by the conditions of life something much more complex. They comprehend the sum of all the conditions under whose sway a plant, an animal, or man, is formed and grows as germ, embryo, youth, and adult. To make a selection from these conditions, to admit some and take them into consideration, to reject and exclude the rest, is evidently an entirely arbitrary procedure. The consideration of only a certain period of life, the neglect of the whole intra-ovarian or intra-uterine period, deserves the same reproach. From this point of view, the existence of a being cannot be severed, any more than the conditions of life under whose rule this existence is accomplished.

A number of cases do away with all doubt as to the action of the conditions of life upon the germ, or upon the embryo, however much it may appear to be protected by the envelopes of the ovum, or by the tissues of the mother. The two Geoffroy Saint-Hilaire have clearly proved that monstrosity dates from the earliest stages of the formation of the being, and indicates in certain cases the external causes which have produced it. The experiments of M. Dareste have confirmed and enlarged in a singular manner these first conclusions, while giving them greater precision. By mixing madder with the food of a female mammal, Flourens produced a red colour in the bones of the fœtus. By placing the eggs of a salmon-trout in waters which only nourished white-trout, Coste noticed the eggs become gradually paler, and produce trout which had lost the characteristic colour of their race. In order to increase the height of our excellent small horses of the “camargue” race, it is sufficient to give the mare during the period of gestation a more plentiful diet than that to which she is accustomed in her half-wild state.

Thus it is established in the clearest manner and by exact experiments that the conditions of life, when acting upon the embryo during the intra-uterine or intra-ovarian part of its existence, are capable of producing either the gravest teratological disorders, or simple and slight deviations. We are, therefore, clearly justified in attributing to the same cause modifications which are placed between these extremes according to their importance. To invoke innateness, in order to explain their appearance, is obviously superfluous. We shall connect, therefore, with actions of this kind the appearance of the first spineless Acacia of which we have spoken before, of the first Ancon sheep in Massachusetts in 1791, that of the first Mauchamp sheep in France in 1828, etc.

The Ancon and Mauchamp races are only propagated by human industry. But these sudden deviations from a given type can also extend and multiply their numbers by themselves. It is well-known that South American oxen are descended from a horned Spanish race. Now, in 1770, a hornless ox was produced in Paraguay. In several years, according to d’Azara, this exceptional form had, as it were, invaded several provinces. Nevertheless, the race is far from being in favour, because the absence of horns renders it less liable to be caught by the lasso, so that its destruction was attempted. It was, therefore, evidently propagated spontaneously.