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insignificant; others more essential. 'When we have 

 properly weighed and catalogued these, we shall, I be- 

 lieve, be on the right road toward a correct understand- 

 ing of species, sub-species, and forms. Xone will deny 

 that the characters for distinguishing species should be 

 those which vary the least. A species founded upon char- 

 acters which are known to be variable has a precarious 

 position at best. Plants are constantly varying, some- 

 times in very slight degree, more often by appreciable 

 leaps. If the conditions are favorable to the existence 

 of a variant, it will thrive and possibly perfect itself with 

 reference to its surroundings until it becomes a separate 

 species, but it is also conceivable that some untoward 

 circumstances may prevent its development beyond a 

 certain point, in which case it is but logical to call it 

 something less than a species. 



The amount of difference between even closely resem- 

 bling forms can be measured, and it would seem only 

 proper to regard any two forms as belonging to the same 

 species when they do not differ in one or more of their 

 essential characters. In this connection it is interesting 

 to note the opinion of Prof. T. D. A. Cockerell. " Spe- 

 cies and sub-species," he says, " are units isolated by 

 nature. It is not a matter of taste how many species 

 exist, .though one might imagine so. to read the current 

 botanical literature. Ultimately we shall have to know 

 how many forms stand physiologically isolated from one 

 another, and these will be recognized as true species. 

 Sub-species are similar, except that at some point the 

 isolation is as yet incomplete. The word variety may as 

 well be abandoned as a distinct category, but it is useful 

 when we do not know the proper status of a plant. The 

 ' form ' (forma) is really something different. As I un- 

 derstand it, it expresses a phase existing wholly within 

 specific limits r a result of the variability of the organ- 

 ism, spontaneous or induced by external conditions." 



In the days when the Darwinian interpretation of the 

 origin of species prevailed, and every species was sup- 

 posed to be the summing up of a certain phase of varia- 



