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.
Pages:
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110