FACTS 199 



more and more a very exact and very exacting mechanism. 

 This does not prevent an old man, for example, from becom- 

 ing one-armed. 



Moreover, we should remark that, even in the first periods 

 of life, the conditions under which a young embryo develops 

 itself are little variable. Without speaking of mammals 

 or birds, in which the conditions of development are won- 

 derfully constant, what variations can we observe in the 

 sea water where an urchin's or herring's egg is growing ? 

 There are, at most, a few variations in temperature or salt- 

 ness ; and besides, when these variations pass certain limits, 

 the embryo dies. 



Thus it is generally said, with a degree of exactness which 

 previous considerations allow us to estimate, that the 

 animal is determined in the egg. We know beforehand that 

 the animal which issues from the egg will resemble the 

 animal which gave the egg ; and so the current meaning of 

 the word heredity is not different from that which we have 

 given by strict definition. 



In general, the term heredity is applied to the fact that 

 children resemble their parents (leave aside for the present 

 the difficulties arising from sexual reproduction, which 

 causes the child to issue from two parents). Now this 

 resemblance-heredity results from the structure-heredity of 

 the egg, since this structure-heredity of the egg determines 

 within certain limits the form of the child. But we must 

 at once take notice that structure-heredity is simply a 

 definition. What we call the heredity of the egg is its physico- 

 chemical structure, whereas resemblance-heredity is a 

 biological theorem which daily observation demonstrates, 

 namely, that the structure-heredity of the egg furnished by 

 a being determines in the child of that being a morphological 

 constitution resembling that of the parent, provided death 



