552 Critical Periods in the History of the Earth. [September 
How like to a violent extermination and re-creation! But the 
Quaternary is fortunately not lost, and we see that there has 
been no such wholesale extermination and re-creation, but only 
gradual though comparatively rapid transition. 
Il. Migration One Chief Cause of Change. But what is still 
more important, we are able to trace with something like cer- 
tainty the cause of these rapid changes, and we find that in the 
higher animals, chief among these causes have been migrations, 
— migrations enforced by changes of climate, and migrations 
permitted by changes of physical geography opening gateways 
between regions previously separated by impassable barriers. 
This point is so important that we must dwell upon it. Only 
an outline, however, of some of these migrations and their effects 
on evolution can be given in the present condition of knowledge. 
During Miocene times, as is well known, evergreens, allied to 
those now inhabiting Southern Europe, covered the whole of Eu- 
rope as far north as Lapland and Spitzbergen. In America, 
Magnolias, Taxodiums, Libocedrus, and Sequoias very similar to, 
if not identical with, those now living on the Southern Atlantic 
and Gulf coasts and in California were abundant in Greenland. 
Evidently there could have been no Polar ice-cap at that time, 
and consequently no arctic species unless on mountain tops. Dur- 
ing the latter part of the Pliocene the temperature did not differ 
much from the present; the Polar ice-cap had therefore commenced 
to form, with its accompaniment of arctic species. With the 
coming on of the Glacial epoch, the polar ice and arctic condi- 
tions crept slowly southward, pushing arctic species to Middle 
Europe and Middle United States, and sub-aretic species to the 
shores of the Mediterranean and the Gulf. With the return of 
more genial climate, arctic conditions went slowly northward 
again, and with them went arctic species slowly migrating, gen- 
eration after generation, to their present arctic home. 
Similarly, molluscous shells migrated slowly southward and 
again northward to their present position. But plants and some 
terrestrial invertebrates, such as insects, had an alternative which 
shells had not, namely, that of seeking arctic conditions also up- 
ward on the tops of mountains. Many did so, and were © 
stranded there until now. It is in this way that we account 
for the otherwise inexplicable fact that Alpine species in MI ss 
Europe are similar or even largely identical with those m t 
United States, and also with those now living in arctic poses 
These species were wide-spread all over Europe and the Uni 
