22 INTRODUCTORY. [CHAP. 



remote parts of the southern hemisphere, where they were free 

 from the competition of the higher forms, and met with favourable 

 conditions, they seemed to take a new lease of life, and attained a 

 fulness and variety of development which they had never reached 

 before. As a rule, more or less complete isolation has been a 

 dominant feature of this development; of which the best and most 

 striking instance is that of the marsupials in Australasia. That 

 area may accordingly be called the marsupial evolutionary centre. 

 Scarcely less striking is the instance of the edentates (of which the 

 original derivation is unknown) in South America, where, in 

 company with certain peculiar extinct groups of ungulates, they 

 attained an extraordinary development, both as regards the 

 number of specific, generic, and family types, and likewise in 

 respect of the bodily size of some of its members. This second 

 area may be termed the evolutionary centre of the edentates. A 

 third great centre is constituted by Europe, Asia, and North 

 America, which appear to have been the main developmental 

 centre of the higher mammals, and may accordingly be named the 

 placental evolutionary centre. Two other minor centres are 

 respectively indicated by Madagascar and Africa south of the 

 Sahara : the former as being the headquarters of the lemurs, may 

 appropriately be spoken of as the lemuroid centre, while the 

 great development of the antelopes in Africa suggests the name of 

 the antelopine evolutionary centre for that continent. 



The circumstance that throughout the greater part of North 



America and Europe a very large proportion of the 



of Conine nts y continents are built up of sedimentary strata of 



and Ocean- marine origin, naturally led geologists in the early 



Ba.si.ns. . . . 1 1*1 



days of their science to the conclusion that every 

 part of the land had at one time been deep ocean, and every 

 stretch of ocean dry land. More careful study led, however, 

 to the belief that this idea was not founded on fact, and that 

 although it was perfectly true that what are now continents had 

 been many times under the sea, yet that such areas had never 

 formed abyssal ocean-depths ; and, conversely, that such ocean- 

 depths had never been dry land. In addition to many other lines 

 of evidence, this view of the permanency of continents and ocean- 

 basins is strongly supported by the circumstance that nearly all 



