196 VARIATION AND HEREDITY 



direct reactions to changed conditions, whereas this is not the case in gradual 

 definite variation, although here also a reaction may possibly be fixed which 

 only takes place under unusual conditions. If a colourless or asporogenous 

 species acquired a power of pigment-production or of spore-formation, 

 a new property would be gained instead of an old one lost, as in the 

 above instances. Further, one slight variation may lead to another, and 

 so on until the plant's life-cycle and power of variation are considerably 

 broadened. This has, however, not yet been attained by experimental 

 means, for the changes due to accommodation are strictly limited by the 

 plant's power of reaction, and this cannot be indefinitely increased even 

 when generation after generation is taxed beyond its limits, as for example 

 by exposure to maximal and supra-maximal temperatures. 



Just as a particular demand only awakens certain definite responses, so 

 also does variation usually follow a certain line of development. Thus the 

 asporogenous varieties of yeast do not have their power of growth and of 

 fermentation diminished 1 , and the permanent loss of the power of spore- 

 formation by Bacillus anthracis does not involve any decrease in its virulence. 

 It is, however, quite possible that a diminished activity of growth may 

 accompany the loss of the power of producing pigments or poisons, and 

 a particular line of variation, disadvantageous to the plant, may lead to an 

 organism becoming incapable of withstanding the struggle for existence 

 in nature. For example, the loss of the power of producing spores is 

 certainly a disadvantage. 



1 Hansen, Centralbl. f. Bact., 1895, 2. Abth., I, p. 859. 



