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 th(H)ry. 



The Mutation Theory and Darwinism. — The nmtation 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 aliout 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 



