GENERAL INTKODUCTION. xlix 



allowed, if what is teohaioally known as rejuvenescence 

 occurs at intervals, infusoria multiply indefinitely ; they 

 become, in a sense, immortal. The protoplasm making 

 up the organs and tissues of any two closely related 

 animals is of necessity almost identical. When such 

 animals are mated cross-fertilisation can hardly be said 

 to take place, and if this is repeated again and again the 

 individual cells (the germ-cells included) doubtless degene- 

 rate in very mixch the same way as the unicellular infuso- 

 rians when prevented frou: conjugating. The result in the 

 highly organised multicellular animal is likely to be the 

 same as in the simple unicellular, viz. senile degeneration 

 of the cells forming or destined to form the organs and 

 tissues. 



In these days, when nothing is taken for granted, the 

 baneful influence of gross inbreeding is sometimes doubted, 

 and some are not even satisfied that inbreeding leads to 

 prepotency. It may therefore be well to give some 

 instances of the eifects of inbreeding. 



Bakewell, Webb, Colling, and Bates were among the 

 first advocates of inbreeding-. One of their followers 

 was Amos Cruickshank. After a time Cruickshank dis- 

 covered inbreeding had been carried too far, his fine herd 

 of cattle was threatened with destruction, and he wisely 

 resorted to intercrossing to rejuvenate his degenerating 

 stock by the introduction of fresh blood. Why a little 

 "new blood,'" it may be from a herd living under almost 

 identical conditions with the dwindling one, should have 

 so profound an influence has not yet been explained. 

 When we discover why an exchange of molecules between 

 two infusorians at an interval of about two hundred 

 generations is suflicient to arrest degeneration, we shall, 

 perhaps, understand why the occasional introduction of 

 " new blood," or of the same blood from, say, over the 

 seas, may prove the salvation of a herd. 



There is abundant evidence that inbreeding leads to 

 prepotency. One or two instances may, however, be 

 given. The Dalmatian dog already mentioned is inbred, 

 as indeed are most Dalmatians. When bred with a pedi- 



