roughout
all these ages we find no indication of change of species, and but little,
comparatively, of migration. We thus get an erroneous idea of _the
permanence and stability of specific forms_, due to the period immediately
antecedent to our own being a _period of exceptional permanence and
stability_ as regards climatic and geographical conditions.[48]

_Date of Last Glacial Epoch and its Bearing on the Measurement of
Geological Time._--Directly we go back from this stable period we come upon
changes both in the forms and in the distribution of species; and when we
pass beyond the last glacial epoch into the Pliocene period we find
ourselves in a comparatively new world, surrounded by a considerable number
of species altogether different from any which now exist, together with
many others which, though still living, now inhabit distant regions. It
seems not improbable that what is termed the Pliocene period, was really
the coming on of the glacial epoch, and this is the opinion of Professor
Jules Marcou.[49] According to our views, a considerable amount of
geographical change must have occurred at the change from the Miocene to
the Pliocene, favouring the refrigeration of the northern hemisphere, and
leading, in the way already pointed out, to the glacial epoch whenever a
high degree of excentricity {123} prevailed. As many reasons combine to
make us fix the height of the glacial epoch at the period of high
excentricity which occurred 200,000 years back, and as the Pliocene period
was probably not of long duration, we must suppose the next great phase of
very high excentricity (850,000 years ago) to fall within the Miocene
epoch. Dr. Croll believes that this must have produced a glacial period,
but we have shown strong reasons for believing that, in concurrence with
favourable geographical conditions, it led to uninterrupted warm climates
in the temperate and northern zones. This, however, did not prevent the
occurrence of local glaciation wherever other conditions led to its
initiation, and the most powerful of such conditions is a great extent of
high land. Now we know that the Alps acquired a considerable part of their
elevation during the latter part of the Miocene period, since Miocene rocks
occur at an elevation of over 6,000 feet, while Eocene beds occur at nearly
10,000 feet. But since that time there has been a vast amount of
denudation, so that these rocks may have been at first raised much higher
than we no