At its
close there will be another series of upheavals, culminating in a
great Ice-age, and the remaining stretch of the earth's story, in
which we live, will form the Quaternary Era.
In point of duration these four eras differ enormously from each
other. If the first be conceived as comprising sixteen million
years--a very moderate estimate--the second will be found to
cover less than eight million years, the third less than three
million years, and the fourth, the Age of Man, much less than one
million years; while the Archaean Age was probably as long as all
these put together. But the division is rather based on certain
gaps, or "unconformities," in the geological record; and,
although the breaches are now partially filled, we saw that they
correspond to certain profound and revolutionary disturbances in
the face of the earth. We retain them, therefore, as convenient
and logical divisions of the biological as well as the geological
chronicle, and, instead of passing from one geological period to
another, we may, for the rest of the story, take these three eras
as wholes, and devote a few chapters to the chief advances made
by living things in each era.
Pages:
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245