326 DARWINISM chap. 



of the species or race, yet he clearly saw that it was not 

 always and necessarily advantageous. He says : " The most 

 important conclusion at which I have arrived is, that the mere 

 act of intercrossing by itself does no good. The good 

 depends on the individuals which are crossed diflering slightly 

 in constitution, omng to their progenitors having been sub- 

 jected during several generations to slightly different con- 

 ditions. This conclusion, as we shall hereafter see, is closely 

 connected with various important physiological problems, such 

 as the benefit derived from slight changes in the conditions of 

 life." ^ Mr. Darwin has also adduced much direct evidence 

 pro\ang that slight changes in the conditions of life are 

 beneficial to both animals and plants, maintaining or restoring 

 their vigour and fertility in the same way as a favourable 

 cross seems to restore it.^ It is, I believe, by a careful 

 consideration of these two classes of facts that Ave shall find 

 the clue to the labyrinth in which this subject has appeared 

 to involve us. 



Supposed Eril liesuUs of Close Interhr ceding. 



Just as we have seen that intercrossing is not necessarily 

 good, we shall be forced to admit that close interbreeding is 

 not necessarily bad. Our finest breeds of domestic animals 

 have been thus produced, and by a careful statistical inquiry 

 Mr. George Darwin has shown that the most constant and 

 long- continued intermarriages among the British aristocracy 

 have produced no prejudicial results. The rabbits on Porto 

 Santo are all the produce of a single female ; they have lived 

 on the same small island for 470 years, and they still abound 

 there and appear to be vigorous and healthy (see p. 161). 



We have, however, on the other hand, overwhelming- 

 evidence that in many cases, among our domestic animals and 

 cultivated plants, close interbreeding does produce bad results, 

 and the apparent contradiction may perhaps be explained on 

 the same general principles, and under similar limitations, as 

 were found to be necessary in defining the value of inter- 

 crossing. It appears probable, then, that it is not inter- 

 breeding in itself that is hurtful, but interbreeding ■without 



^ Cross- and Self-Fertilisation , p. 27. 

 " Animals and Plants, vol. ii. p. 145. 



