d which of these {67} two classes
has left its earliest traces. If the interpretation of the gigantic
foot-steps in the colored sandstone of North America, as belonging to the
cursorial birds, is correct, the first appearance of birds falls in the
time between the reptilia and mammalia; otherwise the first mammalia would
have appeared before the first birds. For if we find the first real bones
of birds only in the Jura and in the Chalk-formation, they are birds with
tail-spines and with teeth in the beak--hence still related to the reptilia
or the sauria. The first traces of mammalia to be found in the Upper Keuper
formation, and in the Jura, belong to the order of opossums or marsupialia;
_i.e._, to that order which (excepting the echidna and the ornithorhynchus
that, as so-called monotremeta, stand the very lowest in the class of the
mammalia, but are very scarce) occupies the lowest stage among the
multitude of mammalia. Only after them do the higher orders of mammalia
appear; and last of all organisms, man.

If we follow more in detail the appearance of the single organisms, some
remarkable modifications show themselves in the course of their appearance
and growth. We have heretofore mentioned the possibility of the appearance
of the mammalia before the bird. Another fact which deserves attention is,
that frequently the lowest representatives of a class or an order do not at
first appear where the highest representatives of the next lower class or
order are in existence, but with lower representatives of a preceding class
or order, viz.: such representatives of the same as are still less
differentiated and unite in themselves comparatively still more generic and
less specific characteristics--as for instance, the lowest and {68}
earliest amphibia, which do not appear at the same time and place with the
most highly organized fishes, but with fishes of still lower organization.
Moreover many groups of organisms show in earlier geological periods a
richness of development from which they have now fallen far away. For
instance, among the mammalia the pachydermata, among the reptilia the
salamander and newt, among the articulata the cephalopoda, are at present
remarkably reduced;--compare with the legions of ammonites and belemnites
of the secondary period the small number of nautilus and cuttle-fish of the
seas at the present day. A similar fortune was experienced by the ferns and
club-mosses which formed whole forests in