800 PRESIDENTS ADDRESS. 



diverge, they intersect the circles belonging to other species, say 

 at A for example. In other words, a plant at A partakes of 

 certain characters both of gracilis and odorata. A systematist 

 may think it his duty to interpose a species at A, as he is unable 

 to recognise it as a form of gracilis more than odorata. This 

 does but alter the position of the difficulty, as further discoveries 

 bring to light fresh circles which invade other spheres of influence."^ 

 To put the matter in another way, — where it is found that two 

 species have affinities, a man may, by unduly concentrating his 

 attention on one, imagine that the forms of both may be looked 

 upon as forms of one. For example, it is quite easy, by a kind of 

 induction, to make all specimens of odorata and gracilis forms of 

 one or other species only. This is where the personal equation 

 comes in, and to quote Darwin, " the opinion of naturalists having 

 sound judgment and wide experience seems the only guide to 

 follow." 



A further remark of Darwin's is much to the point : — 

 " Hence it is the most flourishing, or, as they may be called, 

 the dominant species — those which range widely, are the most 

 diffused in their own country, and are the most numerous in 

 individuals — which most often produce well-marked variecies, or, 

 as I shall consider them, incipient species." Such species as 

 Eucalyptus Gunnii, viminalis, tereticornis, and amygdalina 

 present many confirmatory illustrations of the truth of Darwin's 

 remarks. In this connection I would invite attention to Darwin's 

 dictum : — 



" Species of the larger genera in each country vary more fre- 

 quently than the species of the smaller genera." 



The perplexing results of attaching names to too many forms 

 of Eucalyptus has been prominently before Mr. A. W. Howitt, with 

 whom I have often discussed the question. He is of opinion that 

 names should only be conferred on "groups," which is an ideal 

 not attainable in practice, since the difficulty of defining a species 

 is only transferred to the " group." 



* This expression is a convenient one, though of course, strictly speaking, 

 there is no " influence "' at all. 



