All known animals at the present day may be divided into some
five or six primary divisions, which are known technically as
"_sub-kingdoms_." Each of these sub-kingdoms [9] may be regarded
as representing a certain type or plan of structure, and all
the animals comprised in each are merely modified forms of this
common type. Not only are all known living animals thus reducible
to some five or six fundamental plans of structure, but amongst
the vast series of fossil forms no one has yet been found--however
unlike any existing animal--to possess peculiarities which would
entitle it to be placed in a new sub-kingdom. All fossil animals,
therefore, are capable of being referred to one or other of the
primary divisions of the animal kingdom. Many fossil groups have
no closely-related group now in existence; but in no case do
we meet with any grand structural type which has not survived
to the present day.
[Footnote 9: In the Appendix a brief definition is given of the
sub-kingdoms, and the chief divisions of each are enumerated.]
The old types of life differ in many respects from those now
upon the earth; and the further back we pass in time, the more
marked does this divergence become. Thus, if we were to compare
the animals which lived in the Silurian seas with those inhabiting
our present oceans, we should in most instances find differences
so great as almost to place us in another world.
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