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, such as the apparently sudden
introduction of new forms throughout all past time, and the common
occurrence of wholly isolated types, which cannot be explained
in this way.
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