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. This divergence
is the most marked in the Palaeozoic forms of life, less so in
those of the Mesozoic period, and less still in the Tertiary
period. Each successive formation has therefore presented us
with animals becoming gradually more and more like those now in
existence; and though there is an immense and striking difference
between the Silurian animals and those of to-day, this difference
is greatly reduced if we compare the Silurian fauna with the
Devonian; _that_ again with the Carboniferous; and so on till
we reach the present.

It follows from the above that the animals of any given formation
are more like those of the next formation below, and of the next
formation above, than they are to any others; and this fact of
itself is an almost inexplicable one, unless we believe that
the animals of any given formation are, in part at any rate, the
lineal descendants of the animals of the preceding formation,
and the progenitors, also in part at least, of the animals of the
succeeding formation. In fact, the palaeontologist is so commonly
confronted with the phenomenon of closely-allied forms of animal
life succeeding one another in point of time, that he is compelled
to believe that such forms have been developed from some common
ancestral type by some process of "_evolution_." On the other
hand, there are many phenomena