•2o2 THE EVOLUTION THEOKY 



in the ovary of the animal for the cause of this sudden fertility, 

 possibly in chance circumstances \yhich we are unaAvare of and which 

 make the ovary occasionally more producti\-e, possibly liowevei- in the 

 fact that inbreedinii' may haA' e brought about various slight structural 

 ^^a.riations in the animal, and among these some which made the 

 fertilization of tlie abundantly produced o\-a 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-eell or the ovum 

 may make fertilization difficxilt, or maj^ even in-event it. I need only 

 i-eniind you of the interesting experiments in liybridization which 

 Plliiger and Born made with Batrachians nearly thirtj?- years ago, 

 ■which showed that in two nearly relatetl species of frog the ova of the 

 .species A were frequentl}^ 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 (Banu efnuleiita), 

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

 similai-ity in the efl'ectiveness of the sperm lies simply in ' rovigh 

 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 tlian that of the first species. Thus, as experiment 

 has proved, the spermatozoa of K(tiu( fufca fertilize the ova of almost 

 all other related species, for they ha\e the thinnest head and it is at 

 the same time verj- pointed. In this case, thei'efoi'c, it depends upon 

 the microscopic structure of the o\um whether fertilization can take 

 place or not, and we can imagine that similar or perhaps othei- 

 minute variations had taken place in the ova in the case of Nath\isi\is's 

 .sow, and that these matle it difficult for the sperms of boars of 

 the same family to ett'ect fertilization. These variations may have 

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

 were constantlj^ being brought together in the fertilized ova, and 

 thus an}^ unfavourable directions of vai'iations which existed were 

 .strengthened. 



It seems to me that in this way alone can the injurious ett'ect 

 •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 



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