Post 24742
Created:
Modified:
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 ofdifference, 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 genera 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 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 understand these analogies, if species have once existed as varieties, 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 or 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 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 genera. And thus, the forms of life throughout the universe become divided into groups subordinate to groups.
Bears on natural selection–The term used in a wide sense–Geometrical powers of increase–Rapid increase of naturalised animals and plants–Nature of the checks to increase–Competition universal–Effects of climate–Protection from the number of individuals–Complex relations of all animals and plants throughout nature–Struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus–The relation of organism to organism the most important of all relations.