230 THE THEORIES OF EVOLUTION 



Let us now suppose that a chemical substance is in- 

 troduced into the organism or that the organism's 

 mode of nutrition is modified. The resistance of the 

 organism to modifications due to such causes is very 

 great ; still they may in the long run alter to a certain 

 extent the composition of the blood. This is well 

 illustrated by the action of poisonous and medicinal 

 matters and in a lesser degree by the effects of ordi- 

 nary and normal alimentation. 



The substances which, after being introduced into 

 the blood, modify in a specific way every category of 

 organic cells, must also exert a certain influence on 

 the sexual cells, for why should sexual cells constitute 

 one sole exception to the general rule? If the ovum 

 contains, as we suppose, the same substances which 

 are found in other cells of the organism, it will be sus- 

 ceptible to the influence of the same factors. Factors 

 that have a stimulative action and favour the develop- 

 ment of certain organs by determining in them the 

 formation of a certain substance in larger quantities, 

 will also increase the supply of that substance in the 

 ovum and consequently in the organism which will 

 originate in this ovum, and in which, therefore, the 

 relevant organ will be more developed. The oppo- 

 site phenomenon occurs when a substance causes 

 decrease or degeneration of an organ or tissue. 



This general thesis or interpretation of observed 

 facts admits of the transmission of acquired char- 



