CROSS-BREEDING AND INBREEDING 511 



riage of close relatives. The aversion to the marriage of relatives has 

 sometimes gone beyond the Kmits of genetic relationship and has in- 

 vaded the realm of merely legal or conventional relationships, as in 

 England, where it is, or at least once was, illegal to marry one's deceased 

 wife's sister. 



"Only exceptionally, as in the case of the royal families of Egypt 

 and ancient Peru," says Castle, "has the marriage of brother and 

 sister been sanctioned. The underlying reason in such cases was the 

 belief that the family in question constituted a superior race whose 

 members could find no fit mates outside their own number. There 

 was probably no thought that inbreeding itself was beneficial but only 

 the desire to conserve the superior excellence beUeved to reside in cer- 

 tain individuals. The same considerations probably have led to the 

 occasional practice of inbreeding in animal husbandry, viz., the desire 

 to conserve and perpetuate the superiority of particular individuals." 



It appears that Robert Bakewell, a stock-breeder of the eight- 

 eenth century, was the first to show the value of close inbreeding in 

 maintaining a uniform type of sheep and cattle. Bakewell adopted 

 the plan of mating brother with sister or parent with offspring, much 

 to the horror of his neighbors, who considered such a procedure im- 

 moral; but their scruples were soon broken down by the obvious im- 

 provements obtained and the greatly increased revenue that accrued. 

 The practice of inbreeding has been a favorite one for a long time, and 

 many fine breeds of standard character have been produced mainly in 

 this way. 



Opinions among breeders differ as to whether inbreeding if prac- 

 ticed expertly is injurious. Some believe that inbreeding itself in- 

 volves no possible injury; others hold that it is always more or less 

 harmful. In order to settle this question, geneticists have carried out 

 extensive experiments under conditions of rigid control. • Even these 

 do not agree in their results. One group of workers (Crampe and 

 Ritzema-Bos) found after extensive inbreeding of rats that there was 

 a steady falling off in fertility and general health during the first six 

 generations of inbreeding. The material used, however, was a mixed 

 or hybrid stock to start with, a fact that makes a satisfactory conclu- 

 sion difficult. Weismann inbred a race of white mice for twenty-nine 

 generations. In the first ten generations the average number of young 

 was 6.1; in the second ten generations it was 5.6; and in the last nine 

 generations it was 4.2. Again, nothing was known about the genetic 

 constitution of the original parents. 



Recent experiments carried out by Dr. Helen Dean King at the 



