ctual limits, would be almost sure
to extinguish such huge and unwieldy animals as mastodons, mammoths, and
the like, whose power of enduring altered circumstances must have been
small.

[Footnote a: In _Comptes Rendus, Acad. des Sciences_, Fevr. 2, 1857.]

This general replacement of the tertiary species of a country by
others so much like them is a noteworthy fact. The hypothesis of the
independent creation of all species, irrespective of their antecedents,
leaves this fact just as mysterious as is creation itself; that of
derivation undertakes to account for it. Whether it satisfactorily does
so or not, it must be allowed that the facts well accord with that
assumption.

The same may be said of another conclusion, namely, that the geological
succession of animals and plants appears to correspond in a general
way with their relative standing or rank in a natural system of
classification. It seems clear, that, though no one of the _grand types_
of the animal kingdom can be traced back farther than the rest, yet the
lower _classes_ long preceded the higher; that there has been on the
whole a steady progression within each class and order; and that the
highest plants and animals have appeared only in relatively modern
times. It is only, however, in a broad sense that this generalization
is now thought to hold good. It encounters many apparent exceptions and
sundry real ones. So far as the rule holds, all is as it should be upon
an hypothesis of derivation.

The rule has its exceptions. But, curiously enough, the most striking
class of exceptions, if such they be, seems to us even more favorable to
the doctrine of derivation than is the general rule of a pure and simple
ascending gradation. We refer to what Agassiz calls prophetic and
synthetic types; for which the former name may suffice, as the
difference between the two is evanescent.

"It has been noticed," writes our great zooelogist, "that certain types,
which are frequently prominent among the representatives of past ages,
combine in their structure peculiarities which at later periods are only
observed separately in different, distinct types. Sauroid fishes before
reptiles, Pterodactyles before birds, Ichthyosauri before dolphins, etc.
There are entire families, of nearly every class of animals, which
in the state of their perfect development exemplify such prophetic
relations.... The sauroid fishes of the past geological ages are an
example of this kind. Th