The cause of this is, obviously, that the lapse of time,
indicated by the unconformability, has been sufficiently great
to allow of the dying out or modification of many of the older
forms of life, and the introduction of new ones by immigration.
Apart, however, altogether, from these great physical breaks
and their corresponding breaks in life, there are other reasons
why we can never become more than partially acquainted with the
former denizens of the globe. Foremost amongst these is the fact
that an enormous number of animals possess no hard parts of the
nature of a skeleton, and are therefore incapable, under any
ordinary circumstances, of leaving behind them any traces of
their existence. It is true that there are cases in which animals
in themselves completely soft-bodied are nevertheless able to leave
marks by which their former presence can be detected: Thus every
geologist is familiar with the winding and twisting "trails" formed
on the surface of the strata by sea-worms; and the impressions
left by the stranded carcases of Jelly-fishes on the fine-grained
lithographic slates of Solenhofen supply us with an example of how
a creature which is little more than "organised sea-water" may
still make an abiding mark upon the sands of time. As a general
rule, however, animals which have no skeletons are incapable of
being preserved as fossils, and hence there must always have
been a vast number of different kinds of marine animals of which
we have absolutely no record whatever.
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