generation, and the teats being also the external dependencies of the same parts, there is between the number and arrangement of those parts and that of the paps a physical relation, which we should endeavour to investigate. But I here only endeavour to point out the right path, without entering into a discussion; yet I must observe, that numerous productions depend more upon the internal construction of the parts of generation than any other cause. It certainly does not depend upon the quantity of semen emitted, otherwise the horse, stag, ram, and goat, would be more prolific than the dog, cat, and other animals, who produce a great number of young, though they have but very little in proportion to their size; neither does the number of young depend upon the frequency of coition, for once coupling of the hog and the dog is sufficient to produce a great many young; the length of time occupied in the emission has no effect in this respect, for the dog remains long only because he is retained by an obstacle in the conformation of the parts; and though the boar has not this obstacle yet he remains longer coupled than most animals, but no conclusion can be drawn from that in favour of the numerous productions of the sow, since a cock requires not more than an instant to fecundate all the eggs an hen will produce in a month. I shall have occasion to unfold the ideas I have accumulated, with a view to prove that a simple probability, or doubt, when founded on physical relations, produces more light and advantages than all the final causes put together.