eat variety and exhibiting a very
perfect organisation. The mammalia, for example, were long thought to
have first appeared in Tertiary times, where they are represented in
some of the earlier deposits by all the great divisions of the class
fully developed--carnivora, rodents, insectivora, marsupials, and even
the perissodactyle and artiodactyle divisions of the ungulata--as
clearly defined as at the present day. The discovery in 1818 of a single
lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
the generalisation, since either its mammalian character was denied, or
the geological position of the strata, in which it was found, was held
to have been erroneously determined. But since then, at intervals of
many years, other remains of mammalia have been discovered in the
Secondary strata, ranging from the Upper Oolite to the Upper Trias both
in Europe and the United States, and one even (Tritylodon) in the Trias
of South Africa. All these are either marsupials, or of some still lower
type of mammalia; but they consist of many distinct forms classed in
about twenty genera. Nevertheless, a great gap still exists between
these mammals and those of the Tertiary strata, since no mammal of any
kind has been found in any part of the Cretaceous formation, although in
several of its subdivisions abundance of land plants, freshwater shells,
and air-breathing reptiles have been discovered. So with fishes. In the
last century none had been obtained lower than the Carboniferous
formation; thirty years later they were found to be very abundant in the
Devonian rocks, and later still they were discovered in the Upper Ludlow
and Lower Ludlow beds of the Silurian formation.

We thus see that such sudden appearances are deceptive, and are, in
fact, only what we ought to expect from the known imperfection of the
geological record. The conditions favourable to the fossilisation of any
group of animals occur comparatively rarely, and only in very limited
areas; while the conditions essential for their permanent preservation
in the rocks, amid all the destruction caused by denudation or
metamorphism, are still more exceptional. And when they are thus
preserved to our day, the particular part of the rocks in which they lie
hidden may not be on the surface but buried down deep under other
strata, and may thus, except in the case of mineral-bearing deposits, be
altogether out of our reach. Then, again, how large a proportion o