110 J. LeConte—Critical Periods in the History of the Earth. 
rapid changes in organic forms. Here then we have all the 
characteristics of one of the boundaries between the primary 
divisions of time. Here then we have a transition or critical 
period—a period corresponding to one of the lost intervals; 
only in this instance being so recent, and being also less vio- 
lent than the preceding ones, 7 7s not lost. The study of the 
Quaternary, therefore, ought to furnish the key which will 
unlock many of the mysteries which now trouble us. Some o 
the problems which have been or will be explained by study 
of the Quaternary, we will now briefly mention. 
I. Changes of species not sudden.—If the Quaternary were lost 
and we compared the Tertiary rocks with the unconformably 
overlying recent rocks, and the Tertiary mammals with those 
now living, how great and apparently sudden seems the change! 
How like to a violent extermination and re-creation! But the 
Quaternary is fortunately not lost, and we see that there has 
een no such wholesale extermination and re-creation, but only 
gradual though comparatively rapid change. : 
II. Migration one chief cause of change—But what is still 
of Europe 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 Gulfcoast and in California, bundant in Green- 
land. Evidently there could have been no Polar ice-cap at that 
time and therefore no Arctic species unless on mountain tops. 
uring 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 Arctice species. 
With the coming on of the Glacial epoch, the polar ice and Arc- 
tic conditions crept slowly southward, pushing Arctie species 
to middle Europe and Middle United States, and sub-Arctic 
species to the shores of the Mediterranean and the Gulf. 
ith the return of more genial climate, Arctic conditions went 
slowly northward again, and with them went Arctic species 
slowly migrating generation after generation to their present 
Arctic home. 
