Chap. XVII. CONCLUSION. 143 



observers have all been deceived at the expense of much cost 

 and trouble. A male animal may sometimes be paired with his 

 daughter, granddaughter, and so on, even for seven generations, 

 without any manifest bad result ; but the experiment has never 

 been tried of matching brothers and sisters, which is considered 

 the closest form of interbreeding, for an equal number of gene- 

 rations. There is good reason to believe that by keeping the 

 members of the same family in distinct bodies, especially if 

 exposed to somewhat different conditions of life, and by occa- 

 sionally crossing these families, the evil results may be much 

 diminished, or quite eliminated. These results are loss of con- 

 stitutional vigour, size, and fertility ; but there is no necessary 

 deterioration in the general form of the body, or in other good 

 qualities. We have seen that with pigs first-rate animals have 

 been produced after long-continued close interbreeding, though 

 they had become extremely infertile when paired with their 

 near relations. The loss of fertility, when it occurs, seems never 

 to be absolute, but only relative to animals of the same blood ; 

 so that this sterility is to a certain extent analogous with that 

 of self-impotent plants which cannot be fertilised by their own 

 pollen, but are perfectly fertile with pollen of any other plant 

 of the same species. The fact of infertility of this peculiar 

 nature being one of the results of long-continued interbreeding, 

 shows that interbreeding does not act merely by combining 

 and augmenting various morbid tendencies common to both 

 parents ; for animals with such tendencies, if not at the time 

 actually ill, can generally propagate their kind. Although 

 offspring descended from the nearest blood-relations are not 

 necessarily deteriorated in structure, yet some authors 78 believe 

 that they are eminently liable to malformations; and this is 

 not improbable, as everything which lessens the vital powers 

 acts in this manner. Instances of this kind have been recorded 

 in the case of pigs, bloodhounds, and some other animals. 



Finally, when we consider the various facts now given which 

 plainly show that good follows from crossing, and less plainly 



78 ™* is * he conclusion of Prof. 354, some curious evidence on half the 



Devay <Du Danger des Manages Con- cases of a peculiar form of blindness 



sang 1862 p 9* Vircho* ^ quotes, occurring in the offspring from near 



m the 'Deutsche Jahrbucher,' 1863, s. relatious. 



