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!

Among animals, instances of the same kind may be found in every
sub-kingdom. The 'Globigerina' of the Atlantic soundings is identical
with that which occurs in the chalk; and the casts of lower silurian
'Foraminifera', which Ehrenberg has recently described, seem to indicate
the existence at that remote period of forms singularly like those which
now exist. Among the corals, the palaeozoic 'Tabulata' are constructed
on precisely the same type as the modern millepores; and if we turn to
molluscs, the most competent malacologists fail to discover any generic
distinction between the 'Craniae', 'Lingulae' and 'Discinae' of the
silurian rocks and those which now live. Our existing 'Nautilus' has its
representative species in every great formation, from the oldest to the
newest; and 'Loligo', the squid of modern seas, appears in the lias, or
at the bottom of the mesozoic series, in a form, at most, specifically
different from its living congeners. In the great assemblage of annulose
animals, the two highest classes, the insects and spider tribe, exhibit
a wonderful persistency of type. The cockroaches of the carboniferous
epoch are exceedingly similar to those which now run about our
coal-cellars; and its locusts, termites and dragon-flies are closely
allied to the members of the same groups which now chirrup about our
fields, undermine our houses, or