82 MARRIAGE AND HEREDITY 



a defect or a quality may be transmitted from genera- 

 tion to generation in a latent form, and suddenly 

 revive under favourable conditions. Weismann now 

 declares the transmission of germ -plasm, intact, from 

 one generation to another to be a fact, his contention 

 being that in each individual a portion of the specific 

 substance derived from the parents is not used up in 

 the construction of the body of that individual, but 

 is reserved unchanged for the formation of the germ- 

 cells of the succeeding generation. This theory seems 

 to explain the curious principle of throwing back, 

 and it tends at the same time to eliminate from 

 the professional breeder's calculations the so-called 

 principle of inn4iU. The horse or the dog that has 

 been pure -bred for a sufficient number of genera- 

 tions may be counted upon to exhibit no reversion 

 to inferior blood. But a flaw in the animal's pedi- 

 gree within six or eight generations is always a 

 source of danger. It may crop up at any time ; in 

 other words, the animal, although nominally pure, 

 may throw back to some defective ancestor. 



There is every reason to believe that human beings 

 are subject to the same principle of reversion as the 

 lower animals ; but it is obvious that many genera- 

 tions must be closely studied before the law can be 



