In a later section of the book, in which he recurs to the subject, the mocking tone of Diderot's professed submission to the 'teachings of the faith,' only makes the more manifest the real opinion that he holds and desires to get accepted. "May it not be that, just as an individual organism in the animal or vegetable kingdom comes into being, grows, reaches maturity, perishes and disappears from view, so whole species may pass through similar stages? If the faith had not taught us that the animals came from the hands of the Creator just such as they are now, and if it were permissible to have the least uncertainty about their beginning and their end, might not the philosopher, left to his own conjectures, suspect that the animal world (l'animalité) has from eternity had its separate elements confusedly scattered through the mass of matter; that it finally came about that these elements united—simply because it was possible for them to unite; that the embryo thus formed has passed through an infinite number of successive organizations and developments; that it has acquired in turn movement, sensation, ideas, thought, reflection, conscience, sentiments, passions,—signs, gestures, sounds, articulate speech, language—laws, sciences and arts; that millions of years have elapsed between each of these developments; that there are perhaps still new developments to take place which are as yet unknown to us; that there has been or is to be a stationary condition of things; that the being thus developed is passing out of, or will pass out of, that condition by a continual process of decline, in which his faculties will gradually leave him just as they originally came to him; and that he will finally disappear from nature forever, or rather, will continue to exist, but in a form and with faculties wholly unlike those which characterize him in this moment of time?—But religion spares us many wanderings and much labor." Here, of course, we have not only the transformation of species, but also the sketch of a complete system of materialistic and ateleological evolutional philosophy, after the Spencerian fashion. Most of the chapters of Mr. Spencer's elaborate biography of the universe Diderot gives us in outline:—its 'integration of a diffused, incoherent matter,' its 'successive phases of physical, psychical and social development,' its 'equilibration' and resultant 'stationary state,' and finally its 'alternate cycles of evolution and dissolution.'
The passage first quoted, however, seems to me the more interesting of the two, not only because it is more outspoken and free from the veil of ironical piety, but also because it shows clearly the sources and grounds of Diderot's belief in the mutability of species. He had been stimulated to write largely by the recent appearance of the 'Systéme da la Nature' of Maupertuis; but he ignored the embryological line of argument, and rested his conclusion upon the homologies lately made known by Daubenton and dilated upon by Buffon.