De Vries's Dictum 



will have wings shorter than 20 inches and 

 others wings more than 20 inches in length. 

 But the average wing-length of the offspring of 

 the two mutating individuals will be 20 inches. 



So much, then, for the practical difference 

 between a mutation and a fluctuating variation. 

 In Chapter V. we shall discuss the possible 

 causes of the difference. By way of anticipation 

 we may say that the suggestion we shall make is 

 that a mutation is due to some rearrangement 

 in the particles which represent that part of the 

 organism in the fertilised egg, whereas a fluctu- 

 ating variation is caused by variations in the 

 particles themselves. 



De Vries, it should be noted, bases his theory 

 largely on experimental evidence. His dictum 

 is "the origin of species is an object of experi- 

 mental observation." He has, we consider, 

 proved conclusively that among plants mutations 

 sometimes occur, and, further, that in a mutating 

 plant the same mutation tends to occur again and 

 again. This latter is a most important fact, 

 because it goes some way towards overcoming 

 the difficulty urged by Darwin that isolated 

 sports must be swamped by continual crossing 

 with the normal type. If mutations arise in 

 swarms, as De Vries asserts they do, then any 

 particular mutation is likely, sooner or later, to 

 cross with a similar mutation and so be able to 

 perpetuate itself. 



83 



