596 ROLLIN D. SALISBURY 
tion, perhaps especially of the sites where limestone was made. 
At the same time, the rate at which it was formed, the whole earth 
considered, was probably much reduced. 
It would seem, from the series of physical changes sketched, that 
very profound changes in life should have followed, but it must be 
confessed that, in spite of the conditions which it would seem must 
have been favorable for great destruction of life, and for imposing 
great modifications upon that which survived, statistical evidences of 
the changes which followed are less impressive than would have been 
expected. The data at hand do point to extensive migrations, but 
not to the exterminations and profound modifications which might 
have been anticipated. It seems impossible to think that the changes 
of climate which drove musk oxen to Kentucky and Virginia, and 
Arctic plants and reindeer to the lowlands of central Europe and to the 
Mediterranean, were without very profound biologic significance, 
unless the life of the earth had reached a condition of far greater 
stability than that of earlier times, when lesser physical changes seem 
to have produced greater biological changes. 
One of the features of the late Tertiary land life, and especially 
of the floras, seems to have been the great extent to which types were 
mingled. This mingling of tropical or sub-tropical forms with 
temperate and boreal ones seems to have begun as early as the middle 
of the period. The oscillations of climate which marked the Pleisto- 
cene seem to have had a sifting influence upon the migratory forms, 
and to have forced them to special adaptations and habitats as the 
period progressed. This is suggested, for example, by the floras of 
America and Eurasia. Gray pointed out long ago that the forest 
flora of the eastern part of North America is more like that of Japan 
than like that of the western part of our own continent. In Europe, 
the north-south and south-north migration of the floras as ice-sheets 
advanced and receded was interfered with by the east-west mountain 
ranges and by the seas bordering Europe on the south. In eastern 
Asia and America, on the other hand, the back-and-forth migration 
of the floras was facilitated by the greater continuity of land between 
high and low latitudes, and in America at least, by the absence of 
east-west mountain ranges. In the western part of the United States, 
the irregular topography made repeated latitudinal migrations of the 
