l well, the
movements are going on so slowly that they escape observation. Only
here and there, as an attendant on earthquake shocks or other related
movements of the crust, do we find any trace of the upward march which
produces these elevations. Although not a subject for exact
measurements, these features of mountain growth indicate a vast lapse
of time, during which the elevations were formed and worn away.

Yet another and very different method by which we may obtain some
gauge of the depths of the past is to be found in the steps which have
led organic life from its lowest and earliest known forms to the
present state of advancement. Taking the changes of species which have
occurred since the beginning of the last ice epoch, we find that the
changes which have been made in the organic life have been very small;
no naturalist who has obtained a clear idea of the facts will question
the statement that they are not a thousandth part of the alterations
which have occurred since the Laurentian time. The writer is of the
opinion that they do not represent the ten thousandth part of those
vast changes. These changes are limited in the main to the
disappearance of a few forms, and to slight modifications in those
previously in existence which have survived to the present day. So far
as we can judge, no considerable step in the organic series has taken
place in this last great period of the earth's history, although it
has been a period when, as before noted, all the conditions have
combined to induce rapid modifications in both animals and plants. If,
then, we can determine the duration of this period, we may obtain a
gauge of some general value.

Although we can not measure in any accurate way the duration of the
events which have taken place since the last Glacial period began to
wane, a study of the facts seems to show that less than a hundred
thousand years can not well be assumed for this interval. Some of the
students who have approached the subject are disposed to allow a
period of at least twice this length as necessary for the perspective
which the train of events exhibits. Reckoning on the lowest estimate,
and counting the organic changes which take place during the age as
amounting to the thousandth part of the organic changes since the
Laurentian age, we find ourselves in face once again of that
inconceivable sum which was indicated by the physical record.

Here, again, the critics assert that there may hav