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Show 58 SPECIES OF LARGE GENERA CHAP. II. we shall see how this may be explained, and ~ow the lesser differences between varieties will tend to Increase into the greater differences between species. There is one other point which seems to me worth notice. Varieties generally have much restricted ral)?es: 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 ought to be reversed. But there is also reason to b~lieve, that those species which are very closely alhed to other species, and in so far resemble varieties, . often have much restricted ranges. For instance, Mr. I-I. C. Watson 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 ·7 provinces; whereas, the species to which these varieties belong range over 14·3 provinces. So that the acknowledged varieties have very nearly the same restricted average range, as have those very 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. Finally, then, varieties have the same general characters as species, for they cannot be distinguished from species,-except, firstly, by the discovery of intermediate linking forms, and the occurrence of such links cannot affect the actual characters of the forms which they connect; and except, secondly~ by a certain amount of CHAP. II. RESEMBLE VARIETIES. 5D difference, for two forms, if differing very little, are generally ranked as varieties, notwithstanding that intermediate linking forms have not been discovered; but the amount of difference considered necessary to give to two forms the rank of species is quite indefinite. In ?enera having more than the average number of species In any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but unequally, allied toget.her, forming little clusters round certain species. Species v~ry closely allied to other species apparently hav~ restncted ranges. In all these several respects the spe~Ie~ of large genera present a strong analogy with vaneties. And we can clearly understand these analogies, if species have once existed as varieties, and have ~.hus o:iginat~d: whereas, these analogies are utterly Inexplicable If each species has been independently created. We h~ve, also, .seen that it is the most flourishing and dominant speCies of the larger genera which on an average vary most; and varieties, as we shall hereafter see, tend to become converted into new and distinct species. The larger genera thus tend to become larger ; and throughout nature the forms of life which are now ~ominant ten~ to become still more dominant by leavIng many modified and dominant descendants. But by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, tne forms of life throughout the universe become divided into groups subordinate to groups. |