Inbreeding 189 



The young of later generations also died at a much greater rate, 



as shown in the following table 



It is noticeable that the mortality rose suddenly at the same time 

 (1891) that the number of young per litter decreased. Pairing 

 between brother and sister was found to give poorer results than 

 pairing of father and daughter, or mother and son. Thirty- 

 six per cent of the pairs of siblings were infertile, but only 

 21.4 per cent of the unions between parent and child. Pair- 

 ing of young of the same litter gave essentially the same result as 

 pairing of young of different Htters of the same parents. 



The weights of the rats decreased during the time of the experi- 

 ment. The maximum size of a full-grown male was 300 grams 

 in 1 89 1 ; in 1892 few individuals reached the weight of 275 

 grams ; and after six years the weight had declined to 240 grams. 

 Crampe, who inbred rats, found that they became diseased and 

 abnormal after a time, but Ritzema Bos did not find this in his 

 experiments. He thinks that Crampe must have begun with a 

 diseased stock, which became more diseased as a result of the 

 weakening induced by inbreeding. 



In the case of man there are many customs, behef s, and super- 

 stitions regarding the effects of close inbreeding. Races in 

 widely different parts of the world have rules prohibiting mar- 

 riages between relations. It would be interesting to know how 

 such regulations have arisen, since, as Darwin points out, savages 

 have no finer moral or social feehngs to consider, and are not 

 likely to have taken into account the effects on distant progeny ; 

 for it is admitted that if evil efTects are produced they are not 

 likely to appear before several generations have closely inter- 

 married. The abhorrence of incest, that is so pronounced in 

 most races, can scarcely be claimed to be an inherent instinct, as 

 Darwin points out. The Hindus have developed to an equal 



