xlviii GENERAL INTRODUCTION. 



and perpetuate certain varieties, tliongli when carried too 

 far it not only arrests variation, but diminishes vitality and 

 makes for senility ;ind degeneration* 



It is not hard to understand that a new individual, 

 developed from the union of two germ-cells from dif- 

 ferently constituted individuals, should more or less differ 

 from his parents and all their ancestors. Neither is it 

 difficult to understand how the offspring of closely re- 

 lated parents sprung from closely related ancestors should 

 be almost the image of their parents. But it is not so 

 evident why inbreeding should lead to loss of vitality or 

 to senility. The explanation seems to be that the stuff 

 of which animals and plants are made— the living clay or 

 protoplasm— has a limited lease of life, is only capable of 

 giving rise to a limited number of generations unless 

 revived or rejuvenated, unless " fresh blood " is intro- 

 duced. 



Eemarkable evidence of this has recently been forth- 

 coming from amongst the Protozoa — the group which 

 includes the simplest and smallest living animals. Some 

 of the infusorians multiply by simple division. Like a 

 single colourless blood-corpuscle, one divides into two, 

 each again divides, and so on the process goes, but not 

 indefinitely. There comes a time when the process of 

 division is arrested, " senile degeneracy '■■ sets in, and the 

 new individuals, incapable of feeding or growing, soon die. 

 If, however, an exchange of protoplasm is occasionally 



* Inbreeding is often said to lead to "atavism." If atavism and 

 reversion or regression are considered one and the same tliin"-, then I con- 

 sider it a mistake to say inbreeding indnces atavism. Inbreeding ahiiost 

 invariably leads to degeiieratipn, vifluch is qnite a different thing from 

 regression. Cross-breeding, in virtue of reversion, may restore all the 

 vigour and vitality of the lost founders of some of our degraded inbred 

 herds. Neither hybrids nor half-breeds nor mongrels are necessarily 

 degenerale; they are usually intelligent and vigorous enough, while inbred 

 animals may be physically feeble, and mentally on the level of imbeciles or 

 idiots. The individual in an inbred litier of puppies which shows most 

 evidence of reversion is sometimes the only survivor ; he is invariably the 

 most vigorous. In the same way, a horse bearing the ancestral colours is 

 often as hardy as a mule. 



