only the great, predominant groups of animals, M. Agassiz has classified
the “Ages of Nature” as follows: — 1. The primary or Palaeozoic
age, comprising the whole era preceding the new red sandstone,
constituted the reign of fishes. 2. The secondary age, up to the
chalk, constituted the reign of reptiles. 8. The tertiary age was the
reign of mammals; and the modern age, embracing the most perfect
of created beings, is the reign of man.*
A more minute classification would give us, since the first appearance
of organized beings, not less than ten or twelve great groups of
animals specifically independent of one another: so many entire
races have passed away and been successively replaced by others ; thus
changing repeatedly the whole population of the globe.
The fossiliferous strata have been estimated to he eight miles in
thickness. They were formed, like the metamorphic rocks, at the
bottom of the sea, by sedimentary deposits, and afterwards upheaved
in their consolidated form by central, heat. Such a process, doubtless,
must have been very slow: e. g. the hydrographic basin of the Tigris
and Euphrates is 189,000 square miles; and the alluvial deposit along
the course of those rivers, in the centre, is about 32,400 square miles
in extent, The average rate of encroachment on the sea, at their
mouths on the Persian G-ulf, is about a mile in thirty years. During
its season of flood, the Euphrates transports about one-eightieth of
its bulk of solid matter; and the earthy portion carried by the Tigris
past the city of Bagdad, was ascertained by Mr. Ainsworth to be one-
hundredth of its hulk, or about 7150 pounds every hour.f But these
rivers are insignificant compared with the Ganges, which hourly carries
down 700,000 cubic feet of mud; or the Yellow river, in China,
which transports 2,000,000 feet of sediment to the sea. Our own
Mesha-sebe, “ the Father of Waters,” though purer than either of the
rivers we have named, has already formed a delta 30,000 square miles
in extent, and is yearly sweeping to the sea, from his many tributaries,
the enormous amount of 3,702,758,400 cubic feet of solid matter.
Yet, notwithstanding such immense deposits, it has been estimated
that, if the sediment from all the rivers in the world were spread
equally over the floor of the Ocean, it would require 1000 years to
raise its bottom a single foot ; or about 4,0Q0,000 of years to form a
mass equal to that of the fossiliferous rocks: and if, instead of merely
the present extent of the sea, we include the whole surface of the
globe in such estimate, the time required must be extended to 15,000,000
of years. J When we consider that these strata were formed at the
* Agassiz: Principles of Zoology, p. 189.
f Ainsworth: Assyria, Babylonia and Chaldcea; Euphrates Expedition, 1888, p. 111.
J Somerville: Physical Geography.
bottom of the sea, and thence upheaved by the operation of natural
causes; and that in many cases this process has been more than once
repeated; we may claim a very respectable antiquity for our planet,
since such changes musthave required a duration .wholly incalculable.
We have seen that every great geological change was accompanied
by the disappearance of existing species and the introduction of new.
while the present geographical distribution of plants and animals coincides
with the rise of those strata constituting the surface of the globe.
All has been successive and progressive; plants and animals were
produced in regular order, ascending from simple to complex; one
law has prevailed from earth’s foundations to its superficies ; and
thus our present species are autocthonoi, .originating on the continents
or islands where they were first found. Man himself is no exception
to this law; for the inferior races are everywhere “ glebse adscripti.”
Each of these orders of living beings occupied the earth for an appointed
time, and gave way in turn to higher organizations. Fishes
ruled over the primeval waters: as land gradually formed itself, they
made way for the great amphibious reptiles. Just as fishes represent
the first vertebrata of the sea, so reptiles are their earliest representatives
on land. Reptiles presided over the formation of continents, and
next came the birds. As huge reptiles of the sea were succeeded by
the marine mammalia—the cetaceans—so, on the land, when mountain
chains were thrown up and dry plains formed, leaving extensive
marshy borders, monstrous wading birds, which have left but their
footmarks behind them, succeeded the reptiles, and were followed in
their turn by the amphibious mammals. Each epoch of the land, as
of the sea, (whilst our “ earth formed, reformed, and transformed
itself,”) was marked by the appearance of suitable inhabitants, necessary
to the great plan of creation in preparing the globe for the
reception of mankind.
The tertiary formation extends over most of Europe, and comprises
those famous geological basins which are the sites of its principal cities,
London, Paris, and Vienna; while, in America, it embraces nearly all
the level region of th*e Middle and the Southern States. Its fossils
comprise a mixture of marine, fresh-water, and land species, occurring
in such succession as to show extensive alternations of sea and land;
and giving reason to believe that large portions of the present surface
of the land were covered with immense lakes, like Erie or Ontario.
The animals of the tertiary period, while entirely different from those
of the secondary, were similar to those now existing: marine animals
no longer predominated in the creation — the higher orders
of land animals had now appeared. The same advance is visible in
all the great departments of animated nature. Of the radiates, the