REPRODUCTION AND SEX 473 



the experiment for seven more generations, but the only notable 

 general result was a reduction of the fertility by about thirty per 

 cent. Some experimenters, such as Crampe, have found that the 

 inbreeding of rats resulted in disease and abnormality, but this was 

 not observable in equally careful experiments made by Ritzema-Bos. 

 He in-bred rats for thirty generations; lor the first four years 

 (twenty generations) there was almost no reduction in fertility; 

 after that there was a very marked decrease of fertility, an increase 

 in the rate of mortality, and a diminution of size. These and other 

 experiments on mammals suggest that very close in-breeding may 

 be continued for many generations without any observable evil 

 effects and, on the other hand, that there are limits beyond which 

 inbreeding becomes disadvantageous. It is certain that, if there be 

 well-defined hereditary predisposition to disease in the stock, then 

 in-breeding soon spells ruin. 



Extensive experiments by Castle and others (see Proc. Amer. 

 Acad., xli. 731-786) on the inbreeding of the pomace-fly {Droso- 

 phila ampelophild) led to the general result that "inbreeding 

 probably reduces very slightly the productiveness of Drosophila, 

 but the productiveness may be fully maintained under constant 

 inbreeding (brother and sister) if selection be made from the more 

 productive families". 



Some of the histories of domesticated breeds are so well recorded 

 that they may be ranked as carefully conducted experiments, and 

 it seems that some very successful breeds of cattle — such as Polled 

 Angus — have in their early stages of establishment involved 

 extremely close inbreeding. When we examine the pedigrees of 

 famous bulls and stallions, we find in some cases an extraordinarily 

 close consanguinity. Valuable results have often been attained by 

 using the same stallion repeatedly on successive generations. 



From breeding experiments four general results seem to be clear : 

 (i) that progressive results have usually followed mating within a 

 narrow range of relationship; (2) that close inbreeding tends to fix 

 characters, developing "prepotency"; (3) that close inbreeding 

 may go far without any injurious effect on physique; but (4) that, 

 if there be any morbid idiosyncras}^ close in-breeding tends to per- 

 petuate and augment it. 



Darwin paid much attention to the question of inbreeding (see 

 his Variation of Animals and Planh under Domestication), and his 

 general conclusions were : 



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

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

 vigour, and fertility, sometimes accompanied by a tendency to 

 malformation. (2) The evil effects from close interbreeding are 

 difficult to detect, for they accumulate slowh^ and differ much in 

 degree in different species, whilst the good effects which almost 



