CAUSES OF VARIATION 531 
mutants or fluctuating variants and, when the test of breeding 
true was applied, they proved to be mutants. Thus De Vries 
had obtained mutations under experimental conditions and was 
ready to announce the mutation theory. 
The Mutation Theory and Darwinism. — The mutation the- 
ory does not disturb the theory of evolution by natural selection, 
but holds a different view as to the material upon which natural 
selection works. According to Darwinism, most all kinds of 
variations are inheritable, and even though slight at first, they can 
become intensified through generations of selection and finally 
become distinct characters of new species. According to the 
mutation theory, only mutations are inheritable, and they are 
not built up through generations of selection, but arise suddenly, 
in full force, and breed true thereafter. Thus according to the 
mutation theory, new species arise at one bound, and all that nat- 
ural selection has to do is to determine whether they survive or 
perish. If the mutation is such that the new species is well 
adapted to its surroundings, then, according to the law of the 
survival of the fittest, it will survive; otherwise, it will likely 
perish. 
The mutation theory explains a number of the early objections 
to the theory of natural selection. It accounts for the fact that 
species have so many characters which are apparently of no 
value in fitting the individual to live. It is obvious that a 
species may have characters of no importance as well as useful 
ones, if new characters arise at a bound in full force, and their 
presence does not depend upon their having met the test of fitting 
the species to live through generations of selection. The muta- 
tion theory accounts for the absence of intermediate forms or 
so-called connecting links between species. Evolution by mu- 
tations requires less time than evolution by the natural selec- 
tion of fluctuations, and in this way the theory answers the 
objection to the lack of time. 
Causes of Variations. — There are various causes which have 
to do with bringing about variations in both plants and ani- 
mals, many of which are not understood. Numerous varia- 
tions are due directly to differences in food supply, climatic 
conditions, and other external factors to which the individual 
is exposed. Thus a plant well situated in reference to light, so 
that its leaves can make carbohydrates abundantly, is likely to 
