192 PLANT PHYSIOLOGY AND ECOLOGY 



that Bacon, Lamarck, and Saint-Hilaire held that evolution 

 proceeded chiefly from adaptation. Darwin, while recognizing 

 the occurrence of both adaptation and mutation, was led to think 

 that variation was the common method of origin. Henslow and 

 many other Neo-Lamarckians held that adaptation is the uni- 

 versal method, refusing to accept variation and neglecting mutation. 

 De Vries, on the other hand, eliminates adaptation as unable to 

 produce constant forms, points out that variation has never 

 been proved to originate new forms, and consequently regards 

 mutation as the universal process. In these extreme views there 

 is both truth and error. Adaptation, mutation, and hybridation 

 have been proved by experiment to be able to produce new and 

 distinct forms. While similar proof is lacking in the case of 

 variation, it seems probable that it is because no careful experi- 

 ments have yet been made in regard to it. 



207. Origin by adaptation. New forms may originate in 

 nature by adaptation to the physical factors in consequence of 

 invasion into a different habitat, or of a marked change in the 

 same habitat. It is equally clear that they may be produced 

 artificially. In either case the chances that a new form will arise 

 depend almost wholly upon the stability of the original form. 

 An extremely stable form remains essentially the same in all 

 habitats in w^hich it can grow. A very plastic one gives rise to 

 a new form whenever it enters a new habitat or has its own changed. 

 Origin by adaptation occurs universally in the case of plastic 

 species whenever they are placed under different physical factors. 

 The factors that control origin of this sort are the direct ones, 

 water and light. The ways in which plants respond to them 

 by changes in form and structure have already been discussed 

 in the two chapters preceding. A new form arising from adapta- 

 tion is called an ecad. For example, the seeds of a plastic sun 

 plant that has entered a forest develop into a shade ecad, while 

 a prairie species carried into a bog may give rise to a water ecad. 

 A species able to invade successfully two or more different habitats 

 will produce a corresponding number of ecads, provided it is not 

 too stable. 



Since plastic species are the only ones that give rise readily to 

 new forms, it is to be expected that such forms will respond with 

 similar readiness to new conditions. A shade ecad can be made 

 to return to the parent form by transferring its seeds to the sun. 



