president's address. 



799 



alluded to, with the amount of difference in the forms which he 

 is continually stud3dng." 



Then again he states : — 



" Practically when a naturalist can unite by means of inter- 

 mediate links any two forms, he treats the one as a variety of the 

 other; ranking the most common, but sometimes the one first 

 described, as the species, and the other as the variety. But cases 

 of great difficult}^ sometimes arise in deciding whether or not to 

 rank one form as a variety of another, even when they are closely 

 connected by intermediate links ; nor will the commonly-assumed 

 hybrid nature of the intermediate forms always remove the 

 ditiiculty." 



An extensive acquaintance with the genus Eucalyptus shows 

 me that the more one proceeds with a study of it, the more we 

 find barriers break down, variations presenting themselves in the 

 most unexpected ways. Let me illustrate my point by a homely 

 diagram. The centres of the circles are species, we will sa}^ 

 Eucalyptus gracilis and E. odorata, for the sake of argument. 



Around the centre of each, forms accumulate which more and 

 more diverge from the types, as shown by the spreading circles. 

 The plant represented by the circle at Z still belongs to odorata, 

 but it is a good deal removed from the type. As these circles 



