146 Heredity. 



remain, being continually reproduced, and the world of life would 

 present the spectacle of perfect regularity and supreme monotony. 

 But this is true only in theory. So soon as we come to the facts, 

 we find the law is resolved into secondary laws, or it even appears 

 to vanish in the exceptions. Not to speak of the external causes 

 (chance, influence of circumstances) which interfere with the action 

 of heredity, there are interior causes, inherent in heredity itself, 

 which hinder the law from pursuing the simple course from like to 

 like. A moment's reflection will make this plain. 



In the inferior creatures, in which generation takes place without 

 sexual connection, hereditary transmission from the parent to the 

 progeny occurs in a perfectly natural way. This happens in cases 

 of fission, as in Trembley's hydra, or in the Nais, which naturally 

 divide into two or more individuals like themselves ; and also in 

 cases of gemmation, where a bud forms on an animal and is soon 

 itself changed into a new and complete animal. 



But in the higher forms of generation sexual connection is 

 indispensable ; as a struggle necessarily arises between the sexes, 

 each parent tends to produce" its like. Here hereditary transmis- 

 sion can at best produce only a mixed constitution, holding from 

 both parents. ' Clearly,' says De Quatrefages, ' the mathematical 

 law of heredity would be for the parent creature to reproduce itself 

 completely in its progeny. And perhaps this law, absolute though 

 it be, is to be found underlying all natural phenomena, but in 

 every case it is masked by accessory circumstances, by the condi- 

 tions amid which heredity acts. But it does not only rest on 

 theoretical considerations, it rests also on facts. Although subject 

 to profound and continual disturbance, still, if we note all the 

 phenomena which show in individuals a tendency to obey the 

 mathematical law, heredity is found to realize in the aggregate of . 

 each species the result which it fails to realize in isolated indi- 

 viduals. To use a figurative expression, the true meaning of 

 which cannot fail to be apprehended, while it cannot be verified 

 in the whole, it may be in detail.' 



The question is still more complicated when we descend to 

 individual facts. We meet with so many oddities and exceptions, 

 and so many contradictory opinions in explanation of them, that 

 it seems as though, when we pass from theory to practice, all law 



