392 EXPERIMENTAL STUD Y OF INHERITANCE 



without affecting physique, and that it is very useful in fixing 

 character and developing prepotency. It seems equally certain 

 that, if there be any morbid idiosyncrasy, close inbreeding like- 

 wise tends to perpetuate and augment this. The same is doubt- 

 less true in the case of mankind, though here the problem is 

 complicated by social considerations which may be just as im- 

 portant as those of bodily health. But the idea that there can 

 be any objection to the marriage of two healthy cousins who 

 happen to fall in love with one another is preposterous. 



Darwin's Conclusions. — Charles Darwin devoted much at- 

 tention to the question of inbreeding (see especially his Animals 

 and Plants under Domestication), and his conclusions were : 

 (i) " The consequences of close interbreeding carried on for 

 too long a time are, as is generally believed, loss of size, con- 

 stitutional vigour, and fertility, sometimes accompanied by a 

 tendency to malformation"; (2) "The evil effects from close 

 interbreeding are difficult to detect, for they accumulate slowly 

 and differ much in degree in different species, whilst the good 

 effects which almost invariably follow a cross are from the first 

 manifest " ; (3) " It should however, be clearly understood that 

 the advantage of close interbreeding, as far as the retention of 

 character is concerned, is indisputable, and often outweighs the 

 evil of a slight loss of constitutional vigour." 



Experiments. — Weismann inbred mice for twenty-nine genera- 

 tions, and his assistant Von Guaita continued the inbreeding 

 for seven more generations. The general result was a notable 

 reduction of fertility — about 30%. 



Ritzema-Bos inbred rats for thirty generations ; for the first 

 four years (twenty generations) there was almost no reduction of 

 fertility, but in the following generations there was very marked 

 decrease of fertility, increase of mortality, and decrease of size. 

 But there was no disease or abnormality, such as other experi- 

 menters — e.g. Crampe — have observed. It goes without saying 

 that if there is a diseased stock, or rather a stock with an 



