The Principle of Divergence of Character

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.

The principle of Divergence of Character explains how the lesser differences between varieties will tend to increase into the greater differences between species.

Varieties generally have much restricted ranges.

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 believe, that those 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. 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.

In this same catalogue, 53 acknowledged varieties are recorded. 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.

Varieties have the same general characters as species.

Varieties are distinguished from species in a few ways:

1. By the discovery of intermediate linking forms

The occurrence of such links cannot affect the actual characters of the forms which they connect

2. By a certain amount of 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 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.

The most flourishing and dominant species of the larger genera vary the most on an average.

• Varieties tend to become converted into new and distinct species.
• The larger genera thus tend to become larger.

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.