200 THE THEORIES OF EVOLUTION 



create conditions such that the character under obser- 

 vation will answer all the requirements and, above all, 

 how can we interpret phenomena over which we have 

 no control, which we can merely observe when they 

 appear, and which originate under conditions of which 

 we are totally ignorant ? 



Weismann and the other systematic opponents of 

 the Lamarckian theory challenge all the cases of 

 transmission observed in unicellular organisms, such 

 as microbian cultures modified by certain external in- 

 fluences (whose virulence, for instance, has been at- 

 tenuated) , and which transmit their new character to 

 numberless generations. Those experiments, they 

 say, mean nothing, for in unicellular organisms, germ 

 plasm and soma plasm are not yet differentiated. 

 This difference, however, is not very important and 

 we must not establish a comparison between the re- 

 production of protozoa and that of metazoa, but be- 

 tween the cleavage of a unicellular organism and that 

 of the ovum of a multicellular organism. 



From one generation to another the ovum under- 

 goes a large number of cell cleavages, while the uni- 

 cellular organism undergoes only one, and it is quite 

 possible that in the course of ontogenesis a modifica- 

 tion can be transmitted by the ovum to many genera- 

 tions of cells, but disappear before reaching the end 

 of its development. In other words, a modification 

 may be hereditary and be transmitted as is the case 



