How could this really remarkable difference in the fertility of the sow in question be the result of a formative stimulus, exercised by the sperm-cells of the unrelated boar upon the ova of the female animal? If the progeny of the sow had been more fertile than herself, then we should have been at least logically justified in concluding that this was the case, but it is not intelligible that the egg-cells of this mother sow should be increased twice or three times because they were fertilized by a new kind of sperm as they glided from the ovary. The number of ova which are liberated from the ovary depends in the first instance upon the number of mature ova contained in it; and unless we are to make the highly improbable assumption that the crossing with the strange boar had as an immediate result the maturing of a large number of ova, we must look elsewhere than in the ovary of the animal for the cause of this sudden fertility, possibly in chance circumstances which we are unaware of and which make the ovary occasionally more productive, possibly however in the fact that inbreeding may have brought about various slight structural variations in the animal, and among these some which made the fertilization of the abundantly produced ova by the sperm of the related boar less easy, and caused it to fail more frequently. As will be readily understood, I cannot say anything definite on this point, but we know that very slight variations in the sperm-cell or the ovum may make fertilization difficult, or may even prevent it. I need only remind you of the interesting experiments in hybridization which Pflüger and Born made with Batrachians nearly thirty years ago, which showed that in two nearly related species of frog the ova of the species A were frequently fertilized by the sperms of the species B, but not conversely, the ova of the species B by the sperms of A. This is the case, for instance, with the green edible frog (_Rana esculenta_), and the brown grass-frog (_Rana fusca_), and the reason of this dissimilarity in the effectiveness of the sperm lies simply in 'rough mechanical conditions,' in the width of the micropyle of the ovum, and the thickness of the head of the spermatozoon. If each species possesses a micropyle which is exactly wide enough to admit of the passage of the spermatozoon of its own species, another species will only be able to fertilize these eggs if the head of its spermatozoon be not larger than that of the first species. Thus, as experiment has proved, the spermatozoa of _Rana fusca_ fertilize the ova of almost all other related species, for they have the thinnest head and it is at the same time very pointed. In this case, therefore, it depends upon the microscopic structure of the ovum whether fertilization can take place or not, and we can imagine that similar or perhaps other minute variations had taken place in the ova in the case of Nathusius's sow, and that these made it difficult for the sperms of boars of the