Again, almost all the fossiliferous rocks have been laid down in water; and it is a necessary result of this that the great majority of fossils are the remains of aquatic animals.

The remains of air-breathing animals, whether of the inhabitants of the land or of the air itself, are comparatively rare as fossils, and the record of the past existence of these is much more imperfect than is the case with animals living in water.

Moreover, the fossiliferous deposits are not only almost exclusively aqueous formations, but the great majority are marine, and only a comparatively small number have been formed by lakes and rivers.

It follows from the foregoing that the palaeontological record is fullest and most complete so far as sea-animals are concerned, though even here we find enormous gaps, owing to the absence of hard structures in many great groups; of animals inhabiting fresh waters our knowledge is rendered still further incomplete by the small proportion that fluviatile and lacustrine deposits bear to marine; whilst we have only a fragmentary acquaintance with the air-breathing animals which inhabited the earth during past ages.
Lastly, the imperfection of the palaeontological record, due to the causes above enumerated, is greatly aggravated, especially as regards the earlier portion of the earth's history, by the fact that many rocks which contained fossils when deposited have since been rendered barren of organic remains.

The principal cause of this common phenomenon is what is known as "metamorphism"-- that is, the subjection of the rock to a sufficient amount of heat to cause a rearrangement of its particles.