Chap. XVII. EVIL FROM INTERBREEDING. 95 



Knight, &c., 3 have expressed the strongest conviction on the 

 impossibility of long-continued close interbreeding. Those 

 who have compiled works on agriculture, and have associated 

 much with breeders, such as the sagacious Youatt, Low, &c, 

 have strongly declared their opinion to the same effect. 

 Prosper Lucas, trusting largely to French authorities, has 

 come to a similar conclusion. The distinguished German 

 agriculturist Hermann von Nathusius, who has written the 

 most able treatise on this subject which I have met with, 

 concurs ; and as I shall have to quote from 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 him- 

 self 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, ex- 

 cepting 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 per 

 petually fertilised themselves and been thus subjected for long 

 ages to the closest interbreeding, there is not a single species, 

 as far as I can discover, in which the structure ensures self- 

 fertilisation. On the contrary, there are in a multitude of 



3 For Andrew Knight, see A. 227. Sir J. Sebright's Treatise has 

 Walker, on ' Intermarriage,' 1838, p. just been quoted. 



