ell-known
instance, the general resemblance of the organic remains from the
several stages of the Chalk formation, though the species are distinct
in each stage. This fact alone, from its generality, seems to have
shaken Professor Pictet in his belief in the immutability of species.
He who is acquainted with the distribution of existing species over the
globe, will not attempt to account for the close resemblance of distinct
species in closely consecutive formations, by the physical conditions of
the ancient areas having remained nearly the same. Let it be remembered
that the forms of life, at least those inhabiting the sea, have changed
almost simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious vicissitudes
of climate during the pleistocene period, which includes the whole
glacial epoch, and note how little the specific forms of the inhabitants
of the sea have been affected.

On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation
has often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as
I attempted to show in the last chapter, in any one or in any two
formations, all the intermediate varieties between the species which
appeared at the commencement and close of these periods: but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and these
assuredly we do find. We find, in short, such evidence of the slow and
scarcely sensible mutations of specific forms, as we have the right to
expect.

ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.

We have seen in the fourth chapter that the degree of differentiation
and specialisation of the parts in organic beings, when arrived at
maturity, is the best standard, as yet suggested, of their degree of
perfection or highness. We have also seen that, as the specialisation of
parts is an advantage to each being, so natural selection will tend to
render the organisation of each being more specialised and perfect,
and in this sense higher; not but that it may leave many creatures with
simple and unimproved str