164 THE THEORY OF EVOLUTION 



through many successive generations, deriving 

 thousands of individuals from a single pair of pro- 

 genitors, but, under natural conditions, matters are 

 very different. A single plant may produce thou- 

 sands of seeds in a season, though but a very few, or, 

 perhaps only one, can arrive at maturity and repro- 

 duce in its turn. Unless it possesses some very 

 notable advantage, the determination of which in- 

 dividual seedling shall survive, is far more likely to 

 be conditioned by the spot where it happened to 

 germinate, with reference to light, moisture, etc., 

 than by its germinal constitution. It is difficult to 

 see how the laws of proportionate segregation can 

 have any effect under such conditions. Some stu- 

 dents of genetics go so far as to maintain that all 

 evolutionary changes are made by the elimination 

 of factors, and others that species are immutable, ex- 

 cept as new combinations are formed through hybrid- 

 ization. 



It is quite possible that the Mendelian laws are 

 those of ordinary, conservative inheritance, by 

 means of which species are maintained in their 

 normal condition, and yet may not be applicable, 

 when new characters arise through the action of the 

 environment. This is suggested by Tower's experi- 

 ments on potato-beetles, cited above. He says: 

 "In what way is the constitution of the germ cell 

 modified so that the organism shows in subsequent 

 generations a permanent change in its coloration? 

 It has been pointed out to me by Professor Morgan 



