46 Plant Life and Evolution 



vestigations of Loeb in artificial parthenogenesis, 

 in which he has shown that certain chemical stimuli 

 may replace to a certain extent the function of the 

 spermatozoa in normal fertilization. 



SELECTION 



The last, but not the least, factor to consider is 

 selection. After a variation has appeared, whatever 

 may have been its cause, it remains to be seen 

 whether or not such a variation has sufficient po- 

 tency to maintain itself. The deciding factor in 

 the persistence and cumulation of any line of varia- 

 tion is Natural Selection, which is the final arbiter 

 in determining what forms shall survive as species 

 or groups of species. 



