time that there is no sub-kingdom and no class wholly extinct or
without living representatives.

If we descend to the smaller groups, we find that the number of orders
of plants is about two hundred; and I have it on the best authority
that not one of these is exclusively fossil; so that there is
absolutely not a single extinct ordinal type of vegetable life; and it
is not until we descend to the next group, or the families, that we
find types which are wholly extinct. The number of orders of animals,
on the other hand, may be reckoned at a hundred and twenty, or
thereabouts, and of these, eight or nine have no living representatives.
The proportion of extinct ordinal types of animals to the existing
types, therefore, does not exceed seven per cent.--a marvellously small
proportion when we consider the vastness of geologic time.

Another class of considerations--of a different kind, it is true, but
tending in the same direction--seems to have been overlooked. Not only
is it true that the general plan of construction of animals and plants
has been the same in all recorded time as at present, but there are
particular kinds of animals and plants which have existed throughout
vast epochs, sometimes through the whole range of recorded time, with
very little change. By reason of this persistency, the typical form of
such a kind might be called a "persistent type," in contradistinction
to those types which have appeared for but a short time in the course of
the world's history. Examples of these persistent types are abundant
enough in both the vegetable and the animal kingdoms. The oldest group
of plants with which we are well acquainted is that of whose remains
coal is constituted; and as far as they can be identified, the
carboniferous plants are ferns, or club-mosses, or Coniferae, in many
cases generically identical with those now living!