SELECTION 151 



should be ranked as species or as varieties, 

 the opinion of naturalists of sound judg- 

 ment and wide experience is the only guide, 

 yet this lacks unanimity: for example, of 

 the polymorphic genera (i.e. rich in species 

 with a small range of differences) in the 

 British flora alone, Bentham reckons 112 

 species, but Babington 251. Wallace has 

 shown that no certain criterion can be given 

 by which to define his own convenient work- 

 ing categories of Malayan butterflies and 

 moths, viz. variable forms, local forms, sub- 

 species, and representative species. As De 

 Candolle concluded from his monograph on 

 oaks (in which he shows at least two-thirds 

 of his 300 species to be provisional), ;< so 

 long as a genus is imperfectly known and 

 its species founded upon a few specimens' 

 they seem clearly limited; but, :< just as we 

 come to know them better, intermediate 

 forms flow in and doubts as to specific limits 

 augment." The terms variety and species 

 are thus arbitrarily applied to indefinable 

 groups of more or less closely similar in- 

 dividuals. Common species that range wide 

 and are much diffused are those which vary 

 most. The species of the larger genera in 

 each country vary more frequently than the 

 species of the smaller genera. The species 

 of large genera present strong analogies with 



