NATURAL SELECTION 97 



acting each other. If, however, the mixed breed were 

 divided into two sections, and one section was 

 constantly paired with individuals belonging to the one 

 breed, and the other section with individuals belonging 

 to the other breed, then after six or seven generations 

 the progeny of the one section would be true to the 

 type of the one breed, and the progeny of the other 

 section true to the type of the other breed. 



In a case which admits of no evidence from direct 

 observation, and such a case is this, the only method 

 that ought to be adopted by inquiring science is to 

 consider the action of the natural law involved, and 

 if we know the processes by which the law acts, to 

 determine in accordance with its action. Now all 

 observation that has ever been certified declares that 

 the inheritance of individual differences is dependent 

 finally on the inheritance of blood. When we con- 

 sider the constant intrusion and extrusion of variations, 

 caused by the action of marriage, it is impossible to 

 conceive of its being otherwise. We take into account, 

 of course, the unknown play of Nature in regard to 

 individualisation from one generation to another, of 

 which I have already spoken, and the recurrence of 

 individual variations in alternate generations for a 

 limited period, or so long as the amount of blood 

 remaining is sufficiently potential. 



But the law of extrusion holds on its remorseless 



course, weakening in the first few generations, and 



speedily extinguishing, as to potential action at least, 



all individual variations. As far as observation can 



7 



