That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances.
If we confine our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we combine both
into one general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are, in fact, intermediate or
connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals
discovered by him in Attica serve to break down the intervals between
existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the
most distinct orders of mammals; but so many fossil links have been
disentombed that Owen has had to alter the whole classification, and has
placed certain Pachyderms in the same sub-order with ruminants; for
example, he dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions.
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