It is necessary, here, clearly to distinguish two things: the facts and their explanations. In that which concerns observed facts, the errors that M. Maupas endeavors to point out in the descriptions of M. Balbiani appear to me to be capable of a reduction to a matter of so little significance—admitting that it comes at all from error—that if I had not been apprised of it, I should have regarded the researches of the first author as a confirmation in most details of those of the second. It is to be observed, in fact, that M. Maupas gives almost the same description that Balbiani does of the movements of _Paramæcium aurelia_. "I have followed animals of that species a number of times," he says, "during the preparations for copulation. They exhibit at that moment a very great agitation. They are seen to go and come, rapidly changing their direction. They approach and throw themselves against their congeners, halt in front of them, feel them an instant with their cilia, then leave them, assume the most varied positions, and, finally, when two individuals equally ready for union chance to meet each other, they face about by their anterior extremities so that the two bodies come together and join, with the exception of the posterior extremities, along their whole extent; the union is thus definitively effected." Up to this point, let it be observed, our author's description is but a paraphrase of that of M. Balbiani, which we have given on page 69 of our work; and a simple comparison of the two suffices to prove this. The divergences of fact extend, as it seems to me, to the two following points only: The duration of the preliminaries, and the existence of an epidemic of copulation. M. Maupas thinks that the movements in question never last very long, at the most a quarter of or half an hour among individuals that have arrived at karyogamic maturity. Whereas M. Balbiani has observed these same movements last for several days. I do not know which of these two observations is the more exact; in fact, I do not think it necessary to choose between them, since both may be exact, the duration of the phenomenon generally being dependent upon conditions subject to great change, while M. Maupas himself remarks that the ciliate Infusoria in the variability of all their biological phenomena are veritable thermometers of a very great sensibility. However that may be, whether the movements that precede copulation in _Paramæcium_ last a quarter of an hour, half an hour, or several days, that fact does not change their real character. The second divergence relates to the epidemics of copulation in the case of _Paramæcium aurelia_; observed by M. Balbiani and denied by M. Maupas. "All the individuals of a group," says M. Maupas, "are never found simultaneously in this condition. Hence the tentative preliminaries of copulation, that fail in their object and end in the individuals going to seek elsewhere another partner." I confess, I do not understand this