Lnbreeding 189 
1887 1888 1889 1890 1891 1892 
0 ~ 2.63 5.55 17.39 50 41.18 
The young of later generations also died at amuch greater rate, 
as shown in the following table: — 
1887 1888 1889 1890 1891 1892 
3.9 4.4 5.0 36.7 36.4 45.5 
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 litters 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 1891; 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, beliefs, 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 feelings to consider, and are not 
likely to have taken into account the effects on distant progeny ; 
for it is admitted that if evil effects 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 
