OH. Ill] CAUSES OF VARIATION 23 



A more serious source of confusion is that the term is 

 used by some to denote any discontinuous variation 

 arising ' spontaneously,' by others for cases in which 

 the variety differs from the type in several apparently 

 distinct characters, and not only in one, so that the 

 new form constitutes an ' elementary species.' Since 

 in studying heredity it is usually important to con- 

 sider distinct characters separately, it may be per- 

 missible to use the word for the origin of a form 

 differing recognisably from the type and not connected 

 with it by true intermediates. 



It has already been pointed out that very little is 

 accurately known about the causes of variation, and 

 it is not impossible that the different forms of varia- 

 tion have different origins. Most writers agree that 

 the ultimate cause must lie in the action of environ- 

 ment in some form, but as Darwin clearly stated in 

 the Origin of Species the environment may act 

 directly or indirectly. In variation of size for ex- 

 ample, it is clear that the supply of nourishment, 

 etc., during growth may have considerable influence 

 on the size of the adult, and such variation will 

 commonly be continuous owing to the evenly graded 

 action on different individuals. In these cases the 

 action is direct. If, however, Weismann's theory of 

 germ-plasm and body-plasm is correct, such action may 

 affect only the body and not be transmitted to off- 

 spring. It is also possible that the germ-cells may 



