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. Thus it is that corals, crustacea, insects, mollusks, and a few other kinds of lower forms constitute the greater mass of invertebrate palæontological 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 to turn into rock; in the majority of cases, an entombed bone is infiltrated or replaced by various mineral substances so that in time little or nothing of the original thing would remain, though a mold or a cast would persist.
But even if an animal of the past possessed hard structures, it must have satisfied certain limited conditions to have its remains prove serviceable to students of to-day. A dead mammal must fall upon ground that has just the right consistency to receive it; if the soil is too soft, its several parts will be separated and scattered as readily as though it had fallen upon hard ground where it would be torn to pieces by carnivorous animals. The dead body must then be covered up by a blanket of silt or sand like that which would be deposited as the result of a freshet. If a skeleton is too greatly broken up or scattered, it may be difficult or even impossible for its discoverer to piece together the various fragments and assemble them in their original relations.
Very few individuals have been so buried and preserved as to meet the conditions for the formation of an ideal fossil. To realize how little may be left of even the most abundant of higher organisms, we have only to recall that less than a century ago immense herds of bison and wild horses roamed the Western plains, but very few of their skulls or other bones remain to be enclosed and fossilized in future strata of rocks. When we appreciate all these difficulties, both geological and biological, we begin to see clearly why the ancient lines of descent cannot be known as we know the path and mode of embryonic transformation. The wonder is not that the palæontological record is incomplete, but that there is any coherent and decipherable record at all. Yet in view of the many and varied obstacles that must be surmounted by the investigator, and the adverse factors which reduce the available evidence, the rapidly growing body of palæontological facts is amply sufficient for the naturalist to use in formulating definite and conclusive principles of evolution.