d'Alembert: You don't believe, then, in pre-existent germs?
d'Alembert: Ah, how glad I am of that!
Diderot: Such a theory is against reason and experiment; against experiment, since you would seek in vain for these germs in the egg or in most animals before a certain age; against reason, since, although the mind may conceive of matter as infinitely divisible, it is not so in nature, and it is unreasonable to imagine an elephant wholly formed within an atom, and within that elephant another wholly formed, ant so on to infinity.
d'Alembert: But without these pre-existent germs, how can we account for the generation of animals?
Diderot: If you're worried by the question " which came first, the hen or the egg ", it's because you suppose that animals were originally the same as they are now. What madness! We can no more tell what they were originally than what they will become. The tiny worm, wriggling in the mud, may be in process of developing into a large animal; the huge animal, that terrifies us by its size, is perhaps on the way to becoming a worm, is perhaps a particular and transient production of this planet.
d'Alembert: What's that you are saying?
Diderot: I was saying to you . . . But it'll take us away from our original discussion.
d'Alembert: What does that matter? We can get back to it or not, as we please.
Diderot: Will you allow me to skip ahead a few million years in time?
d'Alembert: Why not? Time is nothing for nature.
Diderot: Will you consent to my extinguishing our sun?
d'Alembert: The more readily, since it will not be the first to have gone out.
Diderot: Once the sun has been extinguished what will be the result? Plants will perish, animals will perish, the earth will become desolate and silent. Light up that star once more, and you immediately restore the necessary cause whereby an infinite number of new species will be generated, among which I cannot swear whether, in the course of centuries, the plants and animals we know to-day will or will not be reproduced.
d'Alembert: And why should the same scattered elements coming together again not give the same results?
Diderot: Because everything is connected in nature, and if you imagine a new phenomenon or bring back a moment of the past, you are creating a new world.
Source: Conversation between D'Alembert and Diderot (1769). from Diderot, Interpreter of Nature, translated by Jean Stewart and Jonathan Kemp, International Publishers,1943; Complete dialogue.