CONSANGUINITY 387 



gamous) reproduction — the extreme of in-breeding. The prac- 

 tically important inquiry is in regard to the limits of profitable 

 in-breeding among types which are normally cross-breeders or 

 exogamous. 



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 : 

 (1) " The consequences of close interbreeding carried on for too 

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

 tutional 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 here- 

 ditary predisposition to disease to start with, then the evil 

 results of inbreeding will soon be evident. But the point is, 

 what will happen if the stock be healthy ? 



Extensive experiments by Castle and others on the inbreeding 

 of the pomace-fly, Drosophila ampelophila, led to the general 



