532 THE EVOLUTION THEORY 



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 

 Pfliiger 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 {Rami esculenta), 

 and the brown grass-frog {Rana fusca), and the reason of this dis- 

 similarity 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 same family to effect fertilization. These variations may have 

 arisen as a result of the continued inbreeding, because the same ids 

 were constantly being brought together in the fertilized ova, and 

 thus any unfavourable directions of variations which existed were 

 strengthened. 



It seems to me that in this way alone can the injurious efiect 

 of inbreeding be made intelligible. From both parents identical ids 

 meet in the fertilized ovum, in greater numbers the longer inbreeding 

 continues, for at the maturation of every germ-cell the number 

 of different ids is diminished by a few, and their number must there- 

 fore gradually decrease, and it is conceivable that ultimately it may 



