PLANT AND ANIMAL EVOLUTION 107 



Vries'^ are of more general occurrence among plants than 

 among animals. Such mutations are attributable to sudden 

 alterations of molecular and atomic constitution in the hered- 

 ity-chromatin, or to the altered forms of energy supplied to 

 the chromatin during development. Sensitiveness to the bio- 

 chemical reactions of the physical environment should theo- 

 retically be more evident in organisms like plants which derive 

 their energy directly from inorganic compounds that are con- 

 stantly changing their chemical formulae with the conditions 

 of moisture, of aridity, of temperature, of chemical soil con- 

 tent, than in organisms like animals which secure their food 

 compounds ready-made by the plants and possessing com- 

 paratively similar and stable chemical formulas. Thus a plant 

 transferred from one environment to another may exhibit much 

 more sudden and profound changes than an animal, for the 

 reason that all the sources of plant energy are profoundly 

 changed while the sources of animal energy in a new environ- 

 ment are only slightly changed. The highly varied chemical 

 sources of plant energy are in striking contrast with the com- 

 paratively uniform sources of animal energy which are primarily 

 the starches, sugars, and proteins formed by the plants. 



In respect to character origin, or the appearance of new 

 characters, therefore, plants may in accordance with the de 

 Vries mutation hypothesis exhibit discontinuity or sudden 

 changes of form and function more frequently than animals. 

 In respect to character coordination , or the harmonious relations 

 of all their parts, plants are inferior to animals only in their 

 sole dependence on catalytic chemical messengers, while animal 

 characters are coordinated both through catalytic chemical 

 messengers and through the nervous system. 



In respect to character velocity, or the relative rates of move- 



^ As distinguished from the earlier defined Mutations of Waagcn (see p. 138). 



