ent effects upon the substances of which animals are
built: they have power to change their hair, to change their skin, to
change certain external appendages or ornamentations, and any other of
those ultimate features which naturalists call specific characters.
Now I would ask what there is in the substances out of which class
characters are built that would make them less susceptible to such
external influences than these specific characters. In many instances
the former are more delicate, more sensitive, far more fragile and
transient in their material nature than the latter. And yet never, in
all the chances and changes of time, have we seen any alteration in the
mode of respiration, of reproduction, of circulation, or in any of the
systems of organs which characterize the more comprehensive groups of
the Animal Kingdom, although they are quite as much under the immediate
influence of physical causes as those structural features which have
been constantly changing.

The woody fibre of the Pine-trees has had the same structure from the
Carboniferous age to this day, while their mode of branching and the
forms of their cones and leaves have been different in each period
according to their respective species. The combination of rings, the
structure of the wings, and the articulations of the legs are the same
in the Cockroaches of the Carboniferous age as in those which infest our
ships and our dwellings to-day, while the proportion of their parts is
on quite another scale. The tissue of the Corals in the Silurian age is
identical in chemical combination and organic structure with that of the
Corals of our modern reefs, and yet the extensive researches upon this
class for which we are indebted to Milne Edwards and Haime have not
revealed a single species extending through successive geological ages,
but show us, on the contrary, that every age has had its own kinds,
differing among themselves in the same way as those of the Gulf of
Mexico differ now from those of the Indian Ocean and the Pacific. The
scales of the oldest known fishes in the Silurian beds have the same
microscopic structure as those of their representative types today, and
yet I have never seen a single fossil fish presenting the same specific
characters in the successive geological epochs. The teeth of the oldest
Sharks show the same microscopic structure as those of the present time,
and we do not lack opportunities for comparison, since the former are a