Chap. XIX. 



FOUR LAST CHAPTERS. 



17* 



tility only by crossing them with other individuals of the same 

 species or even of a distinct species. 



On the other hand, long-continued close interbreeding 

 between the nearest relations diminishes the constitutional 

 vigour, size, and fertility of the offspring ; and occasionally leads 

 to malformations, but not necessarily to general deterioration 

 of form or structure. This failure of fertility shows that the 

 evil results of interbreeding are independent of the augment- 

 ation of morbid tendencies common to both parents, though 

 this augmentation no doubt is often highly injurious. Our 

 belief that evil follows from close interbreeding rests to a large 

 extent on the experience of practical breeders, especially of 

 those who have reared many animals of the kinds which can be 

 propagated quickly; but it likewise rests on several carefully 

 recorded experiments. With some animals close interbreeding 

 may be carried on for a long period with impunity by the 

 selection of the most vigorous and healthy individuals; but 

 sooner or later evil follows. The evil, however, comes on so 

 slowly and gradually that it easily escapes observation, but can 

 be recognised by the almost instantaneous manner in which 

 size, constitutional vigour, and fertility are regained when 

 animals that have long been interbred are crossed with a dis- 

 tinct family. 



These two great classes of facts, namely, the good derived 

 from crossing, and the evil from close interbreeding, with the 

 consideration of the innumerable adaptations throughout nature 

 for compelling, or favouring, or at least permitting, the occa- 

 sional union of distinct individuals, taken together, lead to the 

 conclusion that it is a law of nature that organic beings shall 

 not fertilise themselves for perpetuity. This law was first 

 plainly hinted at in 1799, with respect to plants, by Andrew 

 Knight, 1 and, not long afterwards, that sagacious observer Kol- 

 reuter, after showing how well the Malvaceae are adapted for 



1 ' Transactions Phil. Soc.,' 1799, p. 

 202. For Kolreuter, see ' Mem. ' de 

 l'Acad. de St. Pe'tersbourg,' torn, iii., 1809 

 (published 1811), p. 197. In reading 

 C. K. Sprengel's remarkable work, ' Das 

 entdeckte Geheimniss,' &c, 1793, it j s 

 curious to observe how often this wonder- 



fully acute observer failed to understand 

 the full meaning of the structure of the 

 flowers which he has so well described, 

 from not always having before his mind 

 the key to the problem, namely, the 

 good derived from the crossing of dis- 

 tinct individual plants. 



