Geikie has estimated
the existence of the solid crust of the earth at the most as a hundred
million years. The first appearance of the crust must soon have been
succeeded by the formation of the seas, and a long time does not seem to
have been required to cool the seas to such a degree that life became
possible. It is very probable that life originally commenced in the
great seas, and that the forms which are now usually included in the
plankton or floating-life included the very first living beings.
According to Brooks, life must have existed in this floating condition
during long primeval epochs, and evolved nearly all the main branches of
the animal and vegetable kingdom [712] before sinking to the bottom of
the sea, and later producing the vast number of diverse forms which now
adorn the sea and land.
All these evolutions, however, must have been very rapid, especially at
the beginning, and together cannot have taken more time than the figures
given above.
The agency of the larger streams, and the deposits which they bring into
the seas, afford further evidence. The amount of dissolved salts,
especially of sodium chloride, has been made the subject of a
calculation by Joly, and the amount of lime has been estimated by Eugene
Dubois. Joly found fifty-five and Dubois thirty-six millions of years as
the probable duration of the age of the rivers, and both figures
correspond to the above dates as closely as might be expected from the
discussion of evidence so very incomplete and limited.
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