62 SPECIES OP LARGER GENERA. 



difference between varieties, when compared with each other 

 or with their parent species, is much less than that between 

 the species of the same genus. But when we come to 

 discuss the principle, as I call it, of divergence of character, 

 we shall see how this may be explained, and how the lesser 

 differences between varieties tend to increase into the greater 

 differences between species. 



There is one other point which is worth notice. Varieties 

 generally have much restricted ranges. This statement is 

 indeed scarcely more than a truism, for, if a variety were 

 found to have a wider range than that of its supposed parent 

 species, their denominations would be reversed. But there 

 is reason to believe that the species which are very closely 

 allied to other species, and in so far resemble varieties, often 

 have much restricted ranges. For instance, Mr. H. C. Wat- 

 son has marked for me in the well-sifted London Catalogue 

 of Plants (4th edition) sixty-three plants which are therein 

 ranked as species, but which he considers as so closely allied 

 to other species as to be of doubtful value : these sixty -three 

 reputed species range on an average over 6.9 of the provinces 

 into which Mr. Watson has divided Great Britain. Now, in 

 this same catalogue, fifty-three acknowledged varieties are 

 recorded, and these range over 7.7 provinces ; whereas, the 

 species to which these varieties belong range over 14.3 prov- 

 inces. So that the acknowledged varieties have nearly the 

 same restricted average range, as have the closely allied 

 forms, marked for me by Mr. Watson as doubtful species, 

 but which are almost universally ranked by British botanists 

 as good and true species. 



SUMMARY. 



Finally, varieties cannot be distinguished from species, — 

 except, first, by the discovery of intermediate linking forms; 

 and, secondly, by a certain indefinite amount of difference 

 between them ; for two forms, if differing ve: r little, are 

 generally ranked as varieties, notwithstanding that they 

 cannot be closely connected ; but the amount of difference 

 considered necessary to give to any two forms the rank of 

 species cannot be defined. In genera having more than the 

 average number of species in any country, the species of 

 these genera have more than the average number of varie- 

 ties. In large genera the species are apt to be closely but 

 unequally allied together, forming little clusters round other 



