Chap. XVII. EVIL FEOM INTERBREEDING. 117 



this treatise, I may state that Nathusius is not only intimately 

 acquainted with works on agriculture in all languages, and 

 knows the pedigrees of our British breeds better than most 

 Englishmen, but has imported many of our improved animals, 

 and is himself an experienced breeder. 



Evidence of the evil effects of close interbreeding can most 

 readily be acquired in the case of animals, such as fowls, 

 pigeons, &c, which propagate quickly, and, from being kept in 

 the same place, are exposed to the same conditions. Now I 

 have inquired of very many breeders of these birds, and I have 

 hitherto not met with a single man who was not thoroughly 

 convinced that an occasional cross with another strain of the 

 same sub-variety was absolutely necessary. Most breeders of 

 highly-improved or fancy birds value their own strain, and are 

 most unwilling, at the risk, in their opinion, of deterioration, 

 to make a cross. The purchase of a first-rate bird of another 

 strain is expensive, and exchanges are troublesome; yet all 

 breeders, as far as I can hear, excepting those who keep large 

 stocks at different places for the sake of crossing, are driven 

 after a time to take this step. 



Another general consideration which has had great influence 

 on my mind is, that with all hermaphrodite animals and plants, 

 which it might have been thought would have perpetually ferti- 

 lised themselves, and thus have been subjected for long ages to 

 the closest interbreeding, there is no single species, as far as I can 

 discover, in which the structure ensures self-fertilisation. On the 

 contrary, there are in a multitude of cases, as briefly stated in 

 the fifteenth chapter, manifest adaptations which favour or inevit- 

 ably lead to an occasional cross between one hermaphrodite and 

 another of the same species ; and these adaptive structures are 

 utterly purposeless, as far as we can see, for any other end. 



With Cattle there can be no doubt that extremely close interbreeding 

 may be long carried on, advantageously with respect to external characters 

 and with no manifestly apparent evil as far as constitution is concerned. 

 The same remark is applicable to sheep. Whether these animals have 

 gradually been rendered less susceptible than others to this evil, in order 

 to permit them to live in herds,— a habit which leads the old and vigorous 

 males to expel all intruders, and in consequence often to pair with their 

 own daughters, I will not pretend to decide. The case of BakewelFs Long- 

 horns, which were closely interbred for a long period, has often been 



