334 PLANT-BREEDING 



most common plants of the fields and waste places in Cali- 

 fornia. Immigrants from Europe are of common occurrence, 

 and some plants have spread with a most astonishing rapid- 

 ity. The Napa-tliistle (Centaurea MeHtensis) and the wild 

 chamomile (Matricaria Chamomilla) are the most obvious 

 instances, but many other introduced species could be ad- 

 duced. 



It is evident that such new plants are finding conditions 

 here which suit them as well and perhaps better than those 

 under which they live in Europe. The same phenomena 

 are afforded by other species which have been introduced 

 from America into Europe, and are now common weeds or 

 even dreaded pests with us. The Canadian water pest, or 

 Elodea canadensis and the American Azolla (A. carolinen- 

 sis) are now perhaps the most widely dispersed obnoxious 

 plants of our canals and ditches, occurring in the largest 

 numbers of individuals. 



Such observations are apt to awaken doubts as to the 

 real value of the current ideas concerning the nature of the 

 adaptations of the organisms to their environment. The 

 Napa-thistle and the wild chamomile are evidently as well- 

 fitted for the Californian soil and chmate as any of its own 

 native plants. Notwithstanding this, they have acquired 

 the quahties which enable them to multiply in such stupen- 

 dous numbers here, in another country. Whether in their 

 native localities the soil and the climate were the same, we 

 do not know, but we may confidently assume that their 

 living environment was different, inasmuch as it must have 

 consisted of European plants and animals. And it is gener- 

 ally conceded that living nature has a larger influence on the 

 evolution of new species than the purely physical and chem- 

 ical conditions. 



Our doubt is tliis: Are the native plants of Cahfornia 

 still living under the same influences under which they 



