FOSSILS ARE ONLY THE HARD PARTS. 27 
Asia which have been minutely investigated,—the Cape 
districts and the Himalaya mountains. A series of entirely 
new and very peculiar animal forms have become known to 
us from the rocks of these localities. But we must bear in 
mind that the vast bottom of the existing oceans is at the 
present time quite inaccessible to paleeontological investiga- 
tions, and that the greater part of the petrifactions which 
have lain there from primeval times will either never be 
known to us, or at best only after the course of many 
thousands of years, when the present bottom of the ocean 
shall have become accessible by gradual elevation. If we 
call to mind the fact that three-fifths of the whole surface 
of the earth consists of water, and only two-fifths of land, 
it becomes plain that on this account the palzontological 
record must always present an immense gap. 
But, in addition to these, there exists another series of 
difficulties in the way of paleontology which arises from 
the nature of the organisms themselves. In the first place, 
as a rule only the hard and solid parts of organisms can fall 
to the bottom of the sea or of fresh waters, and be there 
enclosed in the mud and petrified. Hence it is only 
the bones and teeth of vertebrate animals, the calcareous 
shells of molluscs, the chitinous skeletons of articulated 
animals, the calcareous skeletons of star-fishes and corals, 
and the woody and solid parts of plants, that are capable 
of being petrified. But soft and delicate parts, which 
constitute by far the greater portion of the bodies of most 
organisms, are very rarely deposited in the mud under cir- 
cumstances favourable to their becoming petrified, or dis- 
tinctly impressing their external form upon the hardening 
mud. Now, it must be borne in mind that large classes of 
