o observe that, with our increase of
knowledge, the gaps between the older formations become fewer and
smaller; geologists of a few years standing remember how beautifully
has the Devonian system{328} come in between the Carboniferous and
Silurian formations. I need hardly observe that the slow and gradual
appearance of new forms follows from our theory, for to form a new
species, an old one must not only be plastic in its organization,
becoming so probably from changes in the conditions of its existence,
but a place in the natural economy of the district must [be made,] come
to exist, for the selection of some new modification of its structure,
better fitted to the surrounding conditions than are the other
individuals of the same or other species{329}.

{327} _Origin_, Ed. i. p. 312, vi. p. 453.

{328} In the margin the author has written "Lonsdale." This refers
to W. Lonsdale's paper "Notes on the age of the Limestone of South
Devonshire," _Geolog. Soc. Trans._, Series 2, vol. V. 1840, p. 721.
According to Mr H. B. Woodward (_History of the Geological Society
of London_, 1907, p. 107) "Lonsdale's 'important and original
suggestion of the existence of an intermediary type of Palaeozoic
fossils, since called Devonian,' led to a change which was then
'the greatest ever made at one time in the classification of our
English formations'." Mr Woodward's quotations are from Murchison
and Buckland.

{329} <Note in original.> Better begin with this. If species really,
after catastrophes, created in showers over world, my theory false.
<In the above passage the author is obviously close to his theory of
divergence.>

In the Tertiary system the same facts, which make us admit as probable
that new species have slowly appeared, lead to the admission that old
ones have slowly disappeared, not several together, but one after
another; and by analogy one is induced to extend this belief to the
Secondary and Palaeozoic epochs. In some cases, as the subsidence of a
flat country, or the breaking or the joining of an isthmus, and the
sudden inroad of many new and destructive species, extinction might be
locally sudden. The view entertained by many geologists, that each fauna
of each Secondary epoch has been suddenly destroyed over the whole
world, so that no succession could be left for the production of new
forms, is subversive of my theory, but I see no ground