172 THE THEORIES OF EVOLUTION 



Similar phenomena can be observed in every part 

 of the organism: the shape and structure of passive 

 organs are determined by the strain they have to bear ; 

 active organs, like muscles, develop in direct ratio of 

 their activity. 



Thus many structures, which certain theories attrib- 

 ute to heredity, are due, according to Roux, to func- 

 tional stimulation. The activity of tissues and 

 organs, he writes, begins long before birth; muscles 

 form very early ; bones, aponeuroses, ligaments, have 

 at a very early stage to withstand various strains. 

 Still mere activity in the course of one individual 

 existence could not produce complex organs. To pro- 

 duce this result, it takes several generations in the 

 course of which the effects of activity accumulate. 

 This however would be impossible unless the modifi- 

 cations due to functional stimulation could be trans- 

 mitted by inheritance. Roux, therefore, recognises 

 the inheritance of acquired characters as a necessary 

 factor, but he does not offer any physiological expla- 

 nation for it, except as far as chemical modifications 

 are concerned. These may result, he says, from a 

 general condition of organic nutrition which affects 

 the sexual cells. Roux thinks that to morphological 

 characters chemical modifications probably correspond 

 whose action is likely to extend to the reproductive 

 elements. 



Many criticisms can be formulated against almost 



