74 Heredity and Environment 



are usually of two kinds, continuous or slight, and discontinuous 

 or sudden variations. The latter are especially noticeable when 

 variations occur in the normal number of parts, as in four-leaved 

 clover, or six-fingered men, and such numerical variations have 

 been called by Bateson "meristic." However, sudden variations 

 may include any marked departure from the normal type, in color, 

 shape, size, chemical compositions, etc. Such sudden variations 

 have long been known to breeders as "sports," and both Darwin 

 and Galton pointed out the fact that such sports have sometimes 

 given rise to new races or breeds, though Darwin was not in- 

 clined to assign much importance to them in the general process 

 of evolution. Galton, on the other hand, maintained that varia- 

 tions, or what would now be called "continuous variations," can- 

 not be of much significance in the process of evolution, but that 

 the case is quite different with "sports" ("Hereditary Genius," 

 prefatory chapter). 



More recently the entire biological world has been greatly influ- 

 enced by the "Mutation Theory" of deVries, which has placed a 

 new emphasis upon the importance of sudden variations in the 

 process of evolution. At first deVries was inclined to emphasize 

 the degree of difference, that is the discontinuity, in these varia- 

 tions, but in later works this distinction is given a minor place as 

 compared with the question whether variations are inherited or 

 not. Inherited variations, whether large or small, are called by 

 deVries "mutations," whereas non-inherited variations are known 

 as "fluctuations." The former are caused by changes in germinal 

 constitution, the latter by alterations in environmental conditions; 

 the former represent changes in heredity, the latter changes in 

 development. 



.3. Mutations and Fluctuation^. — This clear cut distinction be- 

 tween mutations and fluctuations marks one of the most impor- 

 tant advances ever made in the study of development and evolu- 

 tion. Thousands of fluctuations occur which are purely somatic 

 in character and which do not affect the germ cells, for every 





