Chap. XIX. FOUR LAST CHAPTERS. 159 



fectly healtliy ; and sucli plants can be stimulated to fertility 

 only by being crossed with other individuals of the same or 

 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 de- 

 terioration of form or structure. This failure of fertility 

 shows that the evil results of interbreeding are independent 

 of the augmentation of morbid tendencies common to both 

 parents, though this augmentation no doubt is often highly 

 injurious. Our belief that evil follows from close interbreed- 

 ing rests to a certain extent on the experience of practical 

 breeders, especially of those who have reared many animals 

 of quickl}^ propagating kinds ; but it likewise rests on several 

 carefully recorded experiments. With some animals close 

 interbreeding may be carried on for a long period with im- 

 punity by the selection of the most vigorous and healthy 

 individuals ; but sooner or later evil follows. The evil, how- 

 ever, 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 distinct 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 occasional 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,^ and, not long afterwards, that sagacious 



^ ' Transactions Phil. Soc.,' 1799, observei* failftd to understand the full 

 p. 202. For Kolreuter, see ' Mem. de meaning of the structure of the 

 I'Acad. de St.-Petersbourg,' torn. iii. flowers which he has so well de- 

 1809 (published 1811), p. 197. In scribed, from not always having 

 reading C. K. Sprengel's remarkable before his mind the key to the pro- 

 work, 'Das entdeckte Geheimniss,' blem, namely, the good derived from 

 &c., 1793, it is curious to observe the crossing of distinct individual 

 how often this wonderfully acute plants. 



