As human and animal organization is the most perfect and compounded, their propagation is also the most difficult and least abundant; I here except those animals which, like the fresh-water polypus or worms, are reproduced from their divided parts, as trees are by slips, or plants by their divided roots or suckers; also those which may be found to multiply without copulation; it appears to me that the nature of those have been sufficiently explained in the preceding chapter; and from which, in every kind where an individual produces its resemblance, it is easy to deduce the explanation of the reproduction from expansion and nutrition. But how shall we apply this mode of reasoning to the generation of man and animals distinguished by sexes, and where the concurrence of two individuals is required? We understand, by what has just been advanced, how each individual can produce its like; but we do not conceive how a male and a female produces a third. Before I answer this question, I cannot avoid observing, that all those who have written upon this subject have confined their systems to the generation of man and animals, without paying any attention to other kinds of generation which Nature presents us with, and reproduction in general; and as the generation of man and animals is the most complicated of all kinds, their researches have been attended with great disadvantages, not only by attacking the most difficult point, but also by having no subject of comparison, from which they could draw a solution of the question. To this it is that I principally attribute the little success of their labours; but by the road I have taken we may arrive at the explanation of the phenomena of every kind of generation in a satisfactory manner.