of the birds than
we possess of the reptiles and other tribes; but it must be
remembered, that the evidence of fossils, as to the absence of any
class of animals from a certain period of the earth's history, can
never be considered as more than negative. Animals, of which we
find no remains in a particular formation, may, nevertheless, have
lived at the time, and it may have only been from unfavorable
circumstances that their remains have not been preserved for our
inspection. The single circumstance of their being little liable to
be carried down into seas might be the cause of their
non-appearance in our quarries."--p. 95.

In truth, the researches of geologists are every day bringing to light
new facts, which compel them to modify or abandon many of the positions
they formerly held; so that a considerable portion of the science is a
mere quicksand of shifting theories. We need only allude to the various
suppositions respecting the origin of drift, and to the numerous
modifications of the glacial theory. Important discoveries have been
made within a short time, showing that certain animal tribes had their
origin much farther back than was at first supposed. A few years ago,
reptiles were believed to be the highest type of life that existed
during the era of the new red sand-stone. But Professor Hitchcock's
recent discovery in this stone of the footprints of gigantic birds has
added a higher class to the zooelogy of the period; and within a few
months, in the same red sand-stone of the Connecticut valley, tracks of
two or three species of quadrupeds have been found, some of them being
probably mammifers of a lower grade. It is true, no fossil remains of
these animals have been brought to light; but this want only renders the
discovery more significant for our present purpose. It shows that
certain animals must have lived at the period in question, though their
remains have not yet been found; and from the greater age of the rocks
then formed, and the consequent greater number of convulsions of the
earth's surface to which they have been subjected, these remains may
have entirely disappeared. It is a curious fact, also, that the animal
remains of that period, which have come down to us, belong to genera so
constituted, that their bodies might well survive, if we may so speak,
the shocks which would have destroyed every trace of some more delicate,
or more finely organiz