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by a river whose waters are common to both banks ; in a geographi- 

 cal division of territory the river may be assigned to either the left 

 bank or the right bank, but such an arrangement is arbitrary ; and yet 

 the interior of the countries is unaffected by it. So with the groups 

 of plants ; it cannot be of any possible consequence whether an inter- 

 mediate or frontier plant be assigned to one group or another, and 

 convenience alone should be considered in such a matter. This long 

 since led me to offer the following observations, the truth of which, 

 much more experience entirely confirms : — ' All the groups into which 

 plants are thrown are in one sense artificial, inasmuch as Nature 

 recognizes no such groups. Nevertheless, consisting in all cases of 

 species very closely allied in Nature, they are in another sense natu- 

 ral.'" — Inir. Veg. Kingd. xxx. 



Hence, however, arises the impossibility of so rigidly defining such 

 groups, that their boundaries may at once be recognized by the bota- 

 nist, and equally the impossibility of his declaring of any such oscu- 

 lant species that it absolutely belongs to such or such a gi'oup ; since 

 " mathematical precision is unknown in such subjects, and exceptions 

 occur to all known rules." The only way of dealing with such refrac- 

 tory plants is to ascertain the general tendency of their mass of cha- 

 racters, which will for the most part correspond with particular ones : 

 and thus we obtain an approximation to the station to which such 

 plants will ultimately be found really to belong. 



Linnaeus taught that both genera and species are the work of Na- 

 ture; we believe, with Dr. Lindley, that species only are natural, and 

 that all combinations of species are in one sense artificial : but since 

 in genera those species are (or ought to be) associated which agree with 

 each other in the greatest number of points of structure, such groups as 

 genera are in another sense natural. As in orders, so in genera, there 

 are always certain osculant or connecting forms which it is all but 

 impossible to refer with certainty to any one of those other forms with 

 which, notwithstanding their apparent anomalies, they are evidently 

 most naturally allied. And this must be the case in all groups, large 

 and small, which man has contrived as aids to study ; since, it is evi- 

 dent to every one who studies Nature as she ought to be studied — to 

 every one who seeks to attain something beyond a mere superficial 

 knowledge of her works — that there is truly no such thing as an 

 abrupt transition from one organized being to another. Our own 

 Ray, in the Preface to his ' Historia Plantarum,' long ago remarked 

 that " as Nature never passes from one extreme to another except by 

 something lying between the two, so she is accustomed to produce 



