remains finds about three fifths of the earth's surface under water so
that he cannot explore vast areas of the present ocean beds which were
formerly dry land and the homes of now extinct animals. Thus the field of
investigation is seriously restricted at the outset, but the naturalist
finds his work still 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 palæontologist.

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 palæontological 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.