346 Plant Life and Evolution 



ble ; but that there is necessarily a radical difference 

 between mutations and fluctuating variations has 

 not been satisfactorily proven; and the fact that by 

 artificial selection of slight differences new forms 

 may arise, makes it highly probable that these fluctu- 

 ating variations may also be potent in species form- 

 ing under natural conditions. The occurrence of 

 well-established natural hybrids makes it practically 

 certain that new species sometimes arise directly by 

 the crossing of two well-marked species. 



It also becomes more and more evident that the 

 plant organism is extremely plastic, and readily in- 

 fluenced by changes in the environment, and that the 

 results of such changes may be transmitted to the 

 offspring. The generalized character of the tissues 

 of even the highest plants, shown especially by the 

 study of regeneration, does not support the theory 

 of a special germ-plasm, directly associated with 

 the transmission of hereditary characters. The view 

 that the laws of heredity are exclusively physio- 

 logical is probably too extreme, but on the other 

 hand it does not seem necessary to assume the pres- 

 ence of an infinity of morphological units, "gem- 

 mules," "determinants," etc. It is almost certain 

 that the protoplast does contain many permanent, 

 but invisible, organs, comparable to the nucleus and 

 chromatophores, but the development of the organ- 

 ism probably depends quite as much upon the po- 

 tentialities of these to respond to stimuli, as to their 

 actual form or chemical structure. Whatever 



