indeed, it be assumed that an isthmus separated two seas inhabited by
distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same
exact
periods, a formation in one region often corresponding with a blank
interval in the other, and if in both regions the species have gone on
slowly changing during the accumulation of the several formations and
during the long intervals of time between them; in this case, the
several formations in the two regions could be arranged in the same
order, in accordance with the general succession of the form of life,
and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.
_On the Affinities of extinct Species to each other, and to living
forms_. Let us now look to the mutual affinities of extinct and living
species. They all fall into one grand natural system; and this fact is
at once explained on the principle of descent. The more ancient any
form is, the more, as a general rule, it differs from living forms.
But, as Buckland long ago remarked, all fossils can be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the wide intervals between existing genera,
families, and orders, cannot be disputed. For if we confine our
attention either to the living or to the extinct alone, the series is
far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great pal ontologist, Owen, showing how extinct
animals fall in between existing groups. Cuvier ranked the Ruminants
and Pachyderms, as the two most distinct orders of mammals; but Owen
has discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms
in the same sub-order with ruminants: for example, he dissolves by fine
gradations the apparently
wide difference between the pig and the camel. In regard to the
Invertebrata, Barrande, and a higher authority could not be named,
asserts that he is every day taught that pal ozoic animals, though
248