DARWINISM AND HEREDITY. 39 



stimuli. Any divergence from this normal structure may 

 be due either to some change in the germinal constitution 

 in a constant environment, or to an altered environment 

 acting on an unchanged germinal constitution. The two 

 results differ to some extent ; they should not be called 

 respectively inherited and acquired, the real distinction 

 being that in the one case the new character is called 

 forth by a new stimulus, whereas in the other case the 

 stimulus remains unchanged, but brings forth a new 

 result because it acts on altered germinal factors. This 

 new deviation or change of character, the variation, will 

 reappear in succeeding generations, provided the stimulus 

 be present also. 



That new character (variation) which is due to ger- 

 minal change will always reappear in a constant environ- 

 ment, provided the germinal constitution continues the 

 same. The other new character (variation) will never 

 reappear unless the causal stimulus is also present. If 

 the new stimulus be removed the character necessarily 

 goes also. The real difference between characters which 

 always appear and those which do not is that the former 

 depend on stimuli which are always present, while the 

 latter depend on stimuli which may be absent. 



It follows that there are two quite distinct kinds of 

 rariation for which new names must be found. The 

 name "mutations" may be adopted for differences due 

 to the changes in the inheritance, to the alterations in 

 germinal constitution ; and we may call " modifications " 

 those induced by changes in the factors of the environ- 

 ment. The former are transmitted in the germ-plasm, 

 but not the latter. By mere inspection these two kinds 

 of variations cannot, of course, be recognised ; they can 

 only be distinguished by systematic observation and 

 experiment. A few instances will help to make this 

 clear. 



At first sight nothing could appear more firmly fixed 

 by heredity than the greenness of plants ; for countless 



