VI 
PREFACE 
area during early Tertiary times must have been caused by a 
more abundant supply of warm currents in the Arctic Ocean 
at that time. Consequently North America would probably 
have been more isolated in the north than it is now, for the 
currents must have had ample facilities for passing freely into 
the Arctic Ocean. Palaeontological evidence, however, points 
to an intimate union of North America and the Old World 
during the greater part of the Tertiary Era. There are thus a 
number of interesting problems, all of which are fully dis¬ 
cussed in this volume, quite apart from many other important 
theories arising out of the subject of the origin and distribution 
of life in North America. In this connection the testimony 
of fossil animals, and that of the marine fauna, has been largely 
availed of. To a certain degree the distribution of plants has 
also been included. 
I have endeavoured, as far as possible, to quote all the more 
noteworthy works and papers dealing with the subject under 
consideration. Nevertheless I am conscious of having possibly 
overlooked a few important contributions ; and I only hope my 
colleagues will be good enough to draw my attention to any omis¬ 
sions that they may notice, so as to enable me to add them in 
case a second edition of this book should be called for. I can 
lay no claim to having made careful studies of the literature 
dealing with the Geology or Botany of the American continents. 
The works cited are those that I happened to be acquainted 
with, and no doubt many gaps might be filled by those more 
conversant with these sciences than I am. 
I have hesitated for some time before actually illustrating 
my views as to the former changes of land and water. Some 
geologists consider it inadvisable to publish palaeogeographical 
maps founded on zoogeographical data, since such maps are 
very liable to revision according to the constantly advancing 
state of our knowledge. The latter are also apt to lead to a 
good deal of adverse criticism, for one map can only represent 
a single short phase of a long series of geological ages during 
which repeated changes of land and water probably took 
place. In spite of these objections I have ventured to con¬ 
struct a few of these ideal maps of ancient geography as a 
