Page - 74 - in The Origin of Species

74 ORIGIN OF SPECIES certain forms—that is, round their parent-species? Un- doubtedly there is one most important point of difference between varieties and species; namely, that the amount of 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 dif- ferences 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 in- deed 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) 63 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 63 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, 53 acknowledged varieties are recorded, and these range over 7 "j provinces ; whereas, the species to which these varieties belong range over 14 "3 provinces. So that the ac- knowledged varieties have nearly the same restricted aver- age 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 very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference