CLIMATIC VARIETIES 185 



race should be the actual features which contribute effectively 

 to that result. 



In discussions of geographical distribution in relation to 

 problems of origin it is generally said that very nearly allied 

 species usually occupy distinct areas, while other competent 

 observers state the exact contrary. Lately, for example, Dr. 

 R. G. Leavitt 28 has published an important collection of evidence 

 upholding the latter proposition, taken chiefly from the botanical 

 side, showing how in numerous genera two or more closely allied 

 species coexist, frequently without intermediates, in the same 

 localities, and may even be thus found in company throughout 

 their distribution. The difference of opinion evidently arises 

 from a confusion as to the sense in which the term "species" 

 is understood and applied. Leavitt, for example, is avowedly 

 following Jordan and, among moderns, Sargent, in applying 

 a close analysis, and denoting as species all forms which are 

 distinct and breed true. Against this use of the term I know 

 no valid objection 29 but it must be obvious that if others follow 

 a different practice confusion may result when observations are 

 summarised in general statements. We will consider this subject 

 again in another place, but here it may be sufficient to say that 

 there can scarcely now be a doubt that numbers of these associ- 

 ated species, such as Jordan discriminated, represent various 

 combinations of the presence and absence of Mendelian factors. 

 This does not in any way weaken the argument which Leavitt 

 founds upon the facts, namely, that the observed distribution 

 of these forms is consistent with the supposition of an evolution 

 largely discontinuous. 



On the other hand, those who have come to the opinion that 

 nearly allied species generally occupy distinct ground are pre- 

 sumably more impressed by the characters differentiating the 

 geographically distinct or adaptational races, seeing that genuine 

 intermediates between them are less commonly found. Those 

 geographical races may no doubt contain various differentiated 

 forms; but when all live together, occasional intermediates are 



28 The Geographical Distribution of nearly related Species. Amer. Nat., XLI . 

 1907, p. 207. 



29 See later, p. 242. 



