170 Species
of spreading widely. Throughout a great and open area, not only will there be a better
chance of favourable variations, arising from the large number of individuals of the same
species there supported, but the conditions of life are much more complex from the large
number of already existing species; and if some of these many species become modified
and improved, others will have to be improved in a corresponding degree, or they will be
exterminated. Each new form, also, as soon as it has been much improved, will be able
to spread over the open and continuous area, and will thus come into competition with
many other forms. Moreover, great areas, though now continuous, will often, owing to
former oscillations of level, have existed in a broken condition; so that the good effects
of isolation will generally, to a certain extent, have concurred. Finally, I conclude that,
although small isolated areas have been in some respects highly favourable for the pro-
duction of new species, yet that the course of modification will generally have been more
rapid on large areas; and what is more important, that the new forms produced on large
areas, which already have been victorious over many competitors, will be those that will
spread most widely, and will give rise to the greatest number of new varieties and species.
They will thus play a more important part in the changing history of the organic world.^56Ironically, as Kottler noted, Darwin in the early Notebooks believed that isolation
was a sine qua non for speciation, in part following the views of Leopold von Buch.^57
But by this later stage, Darwin appears to have made Natural Selection the primary
cause of species, requiring that variation needs to occur in situ as it were, and so
needing larger populations to give it opportunity to do so.
He also notes that the structures that are useful in classifying species are often not
of any adaptive value:
Hence modifications of structure, viewed by systematists as of high value, may be
wholly due to the laws of variation and correlation, without being, as far as we can
judge, of the slightest service to the species.^58Considering the traditional logical notions of generic and specific characters from
which the concept of species was drawn by natural historians in the seventeenth cen-
tury, it is interesting to note that in contrast Darwin expects the specific characters to
vary more than the generic, and moreover that if a character is variable in the genus
between closely allied species, it is more variable in the individual species as well:
... on the view that species are only strongly marked and fixed varieties, we might
expect often to find them still continuing to vary in those parts of their structure which
have varied within a moderately recent period, and which have thus come to differ.
Or to state the case in another manner: the points in which all the species of a genus
resemble each other, and in which they differ from allied genera, are called generic
characters; and these characters may be attributed to inheritance from a common pro-
genitor, for it can rarely have happened that natural selection will have modified sev-
eral distinct species, fitted to more or less widely different habits, in exactly the same
manner:—and as these so-called generic characters have been inherited from before
the period when the several species first branched off from their common progenitor,(^56) Op. cit., 80f.
(^57) Kottler 1978, 285–288.
(^58) Da r win 1872, 110.