io6 THE ORIGIN AND EVOLUTION OF LIFE 



the movements of the germ cells. We follow the development 

 of a great variety of automatic migrating organs, especially in 

 the seed and embryonic stages, by which the germs, or chro- 

 matin bearers, are mechanically propelled through the air or 

 water. Plants are otherwise dependent on the motion of the 

 atmosphere and of animals to which they become attached 

 for the migration of their germs and embryos and of their 

 adult forms into favorable conditions of environment. In 

 these respects and in their fundamentally different sources of 

 energy they present the widest contrast to animal evolution. 



In the absence of a nervous system the remarkable actions 

 and reactions to environmental stimuli which plants exhibit 

 are purely of a physicochemical nature. The interactions be- 

 tween different tissues of plants, which become extraordinarily 

 complex in the higher and larger forms, are probably sustained 

 through catalysis and the circulation through the tissues of 

 chemical messengers analogous to the enzymes, hormones (ac- 

 celerators), and chalones (retarders) of the animal circulation. 

 It is a very striking feature of plant development and evolu- 

 tion that, although entirely without the coordinating agency 

 of a nervous system, all parts are kept in a condition of perfect 

 correlation. This fact is consistent with the comparatively 

 recent discovery that a large part of the coordination of animal 

 organs and tissues which was formerly attributed to the ner- 

 vous system is now known to be catalytic. 



Throughout the evolution of plants the fundamental dis- 

 tinctions between the heredity-chromatin and the body-proto- 

 plasm are sustained exactly as among animals. 



It would appear from the researches of de Vries^ and other 

 botanists that the sudden hereditary alterations of plant struc- 

 ture and function which may be known as mutations of de 



* De Vries, Hugo, 1901, 1903, 1905. 



