ife upon this globe very long periods of time.

Through the operation of this law of extinction and of creation,
animated nature, both on the continents and in the seas, has undergone a
marvellous change. In the lias and oolitic seas, the Enaliosauria,
Cetiosauria, and Crocodilia dominated as the Delphinidae and Balaenidae do
in ours; the former have been eliminated, the latter produced. Along
with the cetaceans came the soft-scaled Cycloid and Ctenoid fishes,
orders which took the place of the Ganoids and Placoids of the Mesozoic
times. One after another successive species of air-breathing reptiles
have emerged, continued for their appointed time to exist, and then died
out. The development has been, not in the descending, but in the
ascending order; the Amphitheria, Spalacotheria, Triconodon of the
Mesozoic times were substituted by higher tertiary forms. Nor have these
mutations been abrupt. If mammals are the chief characteristic of the
Tertiary ages, their first beginnings are seen far earlier; in the
triassic and oolitic formations there are a few of the lower orders
struggling, as it were, to emerge. The aspect of animated nature has
altogether changed. No longer does the camelopard wander over Europe as
he did in the Miocene and Pliocene times; no longer are great elephants
seen in the American forests, the hippopotamus in England, the
Rhinoceros in Siberia. The hand of man has introduced in the New the
horse of the Old World; but the American horse, that ran on the great
plains contemporary with the megatherium and megalonyx, has for tens of
thousands of years been extinct. Even the ocean and the rivers are no
exception to these changes.

[Sidenote: Creations and extinctions by law.] What, then, is the manner
of origin of this infinite succession of forms? It is often sufficient
to see clearly a portion of a plan to be able to determine with some
degree of certainty the general arrangement of the whole; it is often
sufficient to know with precision a part of the life of an individual to
guess with probable accuracy his action in some forthcoming event, of to
determine the share he has borne in affairs that are past. It is enough
to appreciate thoroughly the style of a master to ascertain without
doubt the authenticity of an imputed picture. And so, in the affairs of
the universe, it is enough to ascertain the manner of operation of a
part in order to settle the manner of operation of the whole. When,
therefore