more limited, in so far as much of the dry land
itself is not accessible. The perennial snows of the Arctic region render
it impossible to make a thorough search in the frigid zone, and there are
many portions of the temperate and torrid zones that are equally
unapproachable for other reasons. But even where exploration is possible,
the surface rocks are the only ones from which remains can be readily
obtained, for the layers formed in earlier ages are buried so deeply that
their contents must remain forever unknown in their entirety. Only a few
scratches upon the earth's hard crust have been made here and there, so it
is small wonder that the complete series of extinct organisms has not been
produced by the palaeontologist.

A brief survey of the varied groups of animals themselves is sufficient to
bring to light many biological reasons which account for still more of the
vacant spaces in the palaeontological record. We would hardly expect to
find remains of ancient microscopic animals like the protozoa, unless they
possessed shells or other skeletal structures which in their aggregate
might form masses like the chalk beds of Europe. Jellyfish and worms and
naked mollusks are examples of the numerous orders of lower animals having
no hard parts to be preserved, and so all or nearly all of the extinct
species belonging to these groups can never be known. But when an animal
like a clam dies its shell can resist the disintegrating effects of
bacteria and other organic and inorganic agencies which destroy the soft
parts, and when a form like a lobster or a crab, possessing a body
protected by closely joined shell segments, falls to the bottom of the
sea, the chances are that much of the animal's skeleton will be preserved.
Thus it is that corals, crustacea, insects, mollusks, and a few other
kinds of lower forms constitute the greater mass of invertebrate
palaeontological materials because of their supporting structures of one
kind or another. Perhaps the skeletal remains of the vertebrates of the
past provide the student of fossils with his best facts, on account of the
resistant nature of the bones themselves, and because the backboned
animals are relatively modern; then, too, the rocks in which their remains
occur have not been so much altered by geological agencies, or buried so
deeply under the strata formed later. Of course only the hardest kinds of
shells would remain as such after their burial in materials destined t