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Show 58 SPECIES OF LARGE GENERA [Cn.A.P. II. saine genus. But when we come to discuss the pri]J.ciple, as I call it, of Divergence of Character, we shal~ see how this Inay be explained, and how the l.esser 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 ;anges: this statement is indeed scarcely more than a truisn1, for if a variety 'vere found to !lave a. wider ra?ge. than that of its supposed parent-spemes, thmr denominations ought to be Teversed. But there is also reason to believe, that those species which are ve~y ?losely allied to other spec~es, and in so far resen1ble varieties, often have much restncted ranges. FoT instance, Mr. IL ·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 ra~~e on an average over 6.9 of the provinces into which lYlr. Watson has divided Great Britain. Now, in this same catalogue 53 acknowledged varieties are recorded, and these range over 7.7 provinces; whereas, the spec~es to which these varieties belong range over 14.3 prov1nces. So that the acknowledged varieties have very nearly the saine restricted average range, as have those very closely allied forms 1narked for me by Mr. Watson as doubtful species ' but'w hich are almost unive.r sally ranked by Brit- ish botanists as good and true spemes. Finally, then, varieties have the sai.?-e .gen~ral characters as species, for they cannot . be disting~nshed f~om species,-except, firstly, by the discovery of I~termediate linking forms, and the occurrence of such hnks cannot affect the actual characters of the forms which they connect · and except, secondly, by a certain amount of difference: for two forms, if differing very lit~le, are g;ener~lly ranked as varieties, notwithstanding thRt Intermediate hnking- forms have not been discovered; but the amount of dift'erence considered necP.ssary to give to two forms the 0HAP. II.] RESEMBLE VARIETIES. 59 rank of species is quite indefinite. In genera having more th~n 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 together, forming little clusters round certain species. Species very closely allied to other species apparently have restricted ranges. In all these several respects the species of large genera present a strong analogy with varieties. And we can clearly under~ tand these analogies, if species have once existed as v~neties, and have thus originated : whereas, these analogies are utterly inexplicable if each species has been independently created. We have, 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 :o be~ome larger ; and throughout nature the forms of life whiCh are now dominant tend 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 ~enera. And t~u~, th~ forms of life thr?ughout .the universe becoms drv1ded lnto groups sub· ordinate to groups. |
