[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.  These fishes, which preceded the appearance of reptiles, present a combination of ichthyic and reptilian characters not to be found in the true members of this class, which form its bulk at present.  The Pterodactyles, which preceded the class of birds, and the Ichthyosauri, which preceded the Cetaeca, are other examples of such prophetic types."[a]

[Footnote a:  Agassiz, Contributions:  Essay on Classification, p. 117, where, we may be permitted to note, the word “Crustacea” is by a typographical error printed in place of Cetacea.]