U. WHAT IS A SPECIES ? 27 



even a few orders, might be cited as aiding to distinguish 

 the vegetation of Southern England from that of Northern 

 Scothind, by their presence in the former, and absence in 

 the latter. Still, some of the smallest " natural orders," 

 and those least conspicuously distinguished from their 

 allied orders, are as widely spread through the island in 

 the horizontal direction, as are the largest and most 

 clearly distinct. Thus Pingiiiculacece and Callitrichacece 

 extend through the length and breadth of Britain, equally 

 as Filices and Gramina. But this is only a more general 

 or less precise manner of stating the fact, that some spe- 

 cies are thus widely extended. And uncertain as species 

 may appear to be on rigid inquiry, the Phyto-geographer 

 must nevertheless treat them as definite realities, or 

 cease his investigations. 



On the subject of systematic classification, its grades 

 and inequalities, the views of Mr. Bentham are strongly 

 recommended to the consideration of botanists. His 

 clear-sighted " Memorandum on the principles of generic 

 nomenclature " may be seen in the ' Journal of the Lin- 

 nean Society,' — vol. ii. no. 5, June 1857, pages 31, &c. 



2. What is a ' Sj^ecies ' ? 



It has been contended that Orders and Genera are 

 things of human invention,, not of Divine conception ;— 

 that they exist only in books and herbariums, not in na- 

 ture ; that they are simply terms to express the ideas of 

 systematists, in regard to the resemblances between 

 plants ; — that the number of such groups, and the lines 

 of demarcation between them, are conventional in prac- 

 tice, not fixed in nature ; — and that in associating plants 



