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 
