Page:Popular Science Monthly Volume 65.djvu/252

248 theory of Pangenesis. All these ideas are put forward by Maupertuis only as so many likely explanations of facts which, as he insisted, needed more adequate analysis and explanation than the embryological doctrines of the time afforded; his pangenetic theory, in particular, he regarded only as une conjecture bien hardie, mais qui ne serait peut-être pas destitutée de toute vraisemblance.

What is noteworthy in these hypotheses is the group of truths which they involve incidentally. From Maupertuis's exposition of them it is clear that he had been led, by his reflections upon the facts of heredity, to recognize (a) that there is a constant tendency to variation in animals by reason of their double heredity; (b) that there is a further tendency to spontaneous and accidental variations, due in part to mechanical displacements or chance combinations among the ultimate particles of which the embryo is composed; (c) that these variations—and possibly also new characters acquired during the lifetime of the parent —tend to be perpetuated through heredity, provided that they do not unfit the animals that possess them for survival in their environment, and provided also that they are not gradually obliterated through inter-breeding with animals that do not possess them. To one who thus emphasized the factors making for variation and for the conservation of variations, the theory of the mutability of species necessarily appeared more natural than the theory of their fixity. Maupertuis thus passes at once from his theories of heredity to propound the hypothesis that all species may have come from a single primitive pair through the gradual accumulation and transmission of divergent variations. Granting these facts about variation, he writes, "would it not be possible to explain by means of them how the multiplication of the most dissimilar species might be traced back to (aurait pu s'ensuivre de) only two individuals. Such species would have owed their origination merely to the accidental production of certain embryos (à quelques productions fortuites) in which the elementary parts had not retained the arrangement which they had had in the parent animals. Each degree of deviation (erreur) would bring about a new species; and by means of repeated departures from the original form (à force d'ecarts repétés) there would have come about the infinite diversity of animals that we see to-day:—a diversity which may in time increase still further, but to which it may be that the lapse of centuries will bring only imperceptible additions" ('Système de la Nature,' XLV.). In the 'Lettres' Maupertuis again writes, à propos of the inheritance of certain congenital individual variations in the human