Page:Popular Science Monthly Volume 65.djvu/329

Rh outspoken; and in his 'Pensées de l'Interpretation de la Nature' he declared plainly that the doctrine of the mutability of species and of their descent from a common prototype was, if not an established truth, at any rate a legitimate and a necessary working hypothesis for all future biological investigation.

The most interesting and most explicit passage on the subject in the 'Pensées' has, so far as I know, been noted by no English writer; and I therefore translate it without abridgment. "It seems" says Diderot, "that nature has taken pleasure in varying the same mechanism in a thousand different ways. She never abandons any class of her creations before she has multiplied the individuals of it in as many different forms as possible. When one looks out upon the animal kingdom and notes how, among the quadrupeds, all have functions and parts—especially the internal parts—entirely similar to those of another quadruped, would not any one readily believe (ne croirait-on pas volontiers) that there was never but one original animal, prototype of all animals, of which Nature has merely lengthened or shortened, transformed, multiplied or obliterated, certain organs? Imagine the fingers of the hand united and the substance of the nails so abundant that, spreading out and swelling, it envelops the whole—and in place of the human hand you have the foot of a horse. When one sees how the successive metamorphoses of the envelope of the prototype—whatever it may have been—proceed by insensible degrees through one kingdom of Nature after another, and people the confines of the two kingdoms (if it is permissible to speak of confines where there is no real division)—and people, I say, the confines of the two kingdoms with beings of an uncertain and ambiguous character, stripped in large part of the forms, qualities and functions of the one and invested with the forms, qualities and functions of the other—who then would not feel himself impelled to the belief that there has been but a single first being, prototype of all beings? But whether this philosophic conjecture be admitted as true with Doctor Baumann [Maupertuis], or rejected as false with M. de Buffon, it can not be denied that we must needs embrace it (on ne niera pas qu il faille l'embrasser) as a hypothesis essential to the progress of experimental science, to that of a rational philosophy, to the discovery and to the explanation of the phenomena of organic life" (op. cit., XII.). If the rest of the passage left any uncertainty as to the precise nature of the hypothesis that Diderot had in mind, the reference to Maupertuis and Buffon would make his meaning unmistakable.