vi PBEFACE 



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 



