Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period
in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus, the
species which lived at the sixth great stage of descent in the diagram
are the modified offspring of those which lived at the fifth stage,
and are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate
in character between the forms of life above and below. We must,
however, allow for the entire extinction of some preceding forms, and
for the coming in of quite new forms by immigration, and for a large
amount of modification, during the long and blank intervals between
the successive formations. Subject to these allowances, the fauna of
each geological period undoubtedly is intermediate in character,
between the preceding and succeeding faunas. I need give only one
instance, namely, the manner in which the fossils of the Devonian
system, when this system was first discovered, were at once recognised
by palaeontologists as intermediate in character between those of the
overlying carboniferous, and underlying Silurian system.
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