CHAPTER v.— PHYSIOGENESIS. 



BOTANISTS and gardeners are familiar with the 

 effects of physical causes in producing modifica- 

 tions in the characters of plants. That modifications 

 so produced have become hereditary is known to be 

 the fact, and we may therefore infer that the evolution 

 of plant forms has been produced in large degree by 

 similar agencies in past geological ages. Says Hens- 

 low:^ "M. Carriere raised the radish of cultivation, 

 Raphanus saiiviis L. , from the wild species, R. rapha- 

 fiisttim L., and moreover found that the turnip-rooted 

 form resulted from growing it in a heavy soil, and the 

 long-rooted one in a light soil. Pliny records the same 

 fact as practised in Greece in his day, saying that the 

 male (turnip form) could be produced from the female 

 (long form) by growing it in a " cloggy soil." The 

 rule may be laid down that a species [of plant] may 

 be constant as long as its environment is constant, but 

 no longer. I have changed the spiny Ononis spinosa 

 L. , the rest-harrow, both by cuttings and by seed into 

 a spineless form, undistinguishable from the species 

 O. repens L. in two years ; but it would have, I doubt 



I Natural Sciencf, 1894, pp. 239-26*. 



