310 Plant Life and Evolution 



we find it developed from the base of the petiole. 

 The replacing of the primitive leaf by vertical 

 phyllodes in many species of Acacia may be also 

 considered as a case of determinate variation, and 

 the many more that might be cited all tend to show 

 that the essential organization of all the higher 

 plants, at least, is sufficiently alike to produce much 

 the same reactions in response to similar stimuli, 

 so that there is a marked similarity in the structures 

 resulting from adaptation to special conditions in 

 various lines of development. 



Variation Greatest in Lately Developed Char- 

 acters. In his studies on variation, Darwin em- 

 phasizes the fact that variability is much more 

 marked in what one may assume to be later developed 

 characters. Thus generic characters are less varia- 

 ble than specific ones, and wild species are very 

 much less variable than cultivated ones. Just what 

 constitutes a " species " is more or less a matter 

 of personal opinion. How far such varieties as 

 those described by De Vries, which can be main- 

 tained by artificial selection, can be called species, 

 is not an easy matter to decide. Many cultivated 

 races of both plants and animals, which are un- 

 doubtedly variations from some single recognized 

 species, differ far more widely from this and from 

 each other, than is the case among many wild 

 species, or sometimes even genera ; but the purity of 

 such artificial species can be maintained only by 

 careful artificial selection. Even in nature the 



