Darwin and the Darwinians 167
that all the species of a genus are equally distinct from each other; they may generally
be divided into sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around other species. And
what are varieties but groups of forms, unequally related to each other, and clustered
round certain forms—that is, round their parent-species? Undoubtedly 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.^43So Darwin presents species themselves as real, but not as a formal and fixed rank.
Genera are like species and varieties in that they are the result of groupings of varia-
tion. They too do not seem to be a fixed rank, commensurate across all genera. He
summarizes the argument in this chapter thus:
Finally, varieties cannot be distinguished from species,—except, first, by the discov-
ery 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 considered necessary to give to any two forms the rank of species cannot
be defined. 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 other species. Species very closely allied to other species apparently
have restricted ranges. In all these respects the species of large genera present a strong
analogy with varieties. And we can clearly understand these analogies, if species once
existed as varieties, and thus originated; whereas, these analogies are utterly inexpli-
cable if species are independent creations.^44Darwin had no doubt that species were formed through selection on varietal
forms, and this provides the missing mechanism for how conditions of life can give
rise to varieties:
Again, it may be asked, how is it that varieties, which I have called incipient species,
become ultimately converted into good and distinct species, which in most cases obvi-
ously differ from each other far more than do the varieties of the same species? How
do those groups of species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus, arise? All these
results, as we shall more fully see in the next chapter, follow from the struggle for life.
Owing to this struggle, variations, however slight and from whatever cause proceed-
ing, if they be in any degree profitable to the individuals of a species, in their infinitely
complex relations to other organic beings and to their physical conditions of life, will
tend to the preservation of such individuals, and will generally be inherited by the off-
spring. The offspring, also, will thus have a better chance of surviving, for, of the many
individuals of any species which are periodically born, but a small number can survive.
I have called this principle, by which each slight variation, if useful, is preserved, by
the term Natural Selection, in order to mark its relation to man’s power of selection.^45(^43) Op. cit., 49.
(^44) Op. cit., 49f.
(^45) Op. cit., chapter III, p51f.