The famous Boerhaave having asked Leeuwenhoek, if he had not observed in spermatic animals different degrees of growth and size? Leeuwenhoek answered, that having dissected a rabbit, he observed in the semen an infinite number of living animals. "Incredibilem, says he, viventium animalculorum, numerum conspexerunt, cum hæc animalcula scypho imposita vitreo & illic emortua, in rariores ordines disparassent, & per continuos aliquot dies sæpius visu examinassem, quædam ad justam magnitudinem nondum excrevisse adverti. Ad hæc quasdam observavi particulas perexiles & oblongas, alias aliis majores, &, quantum oculis apparebat, cauda destitutas; quas quidem particulas non nisi animalcula esse credidi, quæ ad justam magnitudinem non excrevissent."[V] Here then are animalcules of different sizes, some with tails and others without, which much better agrees with my experiments, than with Leeuwenhoek's own system. We differ only in one particular; he says, that those without tails were young animalculæ, which were not arrived at their full growth; while I, on the contrary, have seen these pretended animals quit the filaments with tails or threads, and afterwards lose them by degrees. [V] See vol. IV. pages 280 and 281. In the same letter to Boerhaave, he says, in the semen of a ram, he perceived animalcules following each other in swarms like a flock of sheep. "A tribus circiter annis testes arietis, adhuc calentes, ad ædes meas deferri curaveram, cum igitur materiam ex epididymibus eductam, ope microscopii contemplarer, non sine ingenti voluptate advertebam animalcula omnia, quotquot innatabant semini masculino, eundem natando cursum tenere, ita nimirum ut quo itinere priora prinatarent eodem posteriora subsequerentur, adeo ut hisce animalculis quasis sit ingenitum, quod oves factitare vidimus, scilicet ut precedentium vestigiis grex universus incedat." This observation, which Leeuwenhoek made in 1713, and which he looks upon as singular and novel, proves to me, that he had never examined the seminal liquors of animals with attention, at least sufficient to give very exact descriptions of them. Leeuwenhoek was sixty-one years old in 1713, had made microscopical observations for more than forty-five years, had published the discovery of spermatic animals for about thirty-six years, and then, for the first time, saw in the seminal liquor of a ram, what is seen in all seminal liquors, and what I have described in Experiment IX. in the seed of a man; Experiment XII. in the seed of a dog; and in Experiment XXIX. in that of a bitch. It is not necessary to suppose the spermatic animals of the ram are endowed with instinct, to explain the floating of these animals, in flocks like sheep, since those of a man, dog, or bitch, does the same; and which motion depends solely on particular circumstances, whose principle is, that all the fluid matter of the seed is on one side, while the thick matter is on the other; for then