EVIDENCES FROM GEOGR/\PHIC DISTRIBUTION 183 



are believed to have been either marsupials or some allied lowly forms. 

 In North America also, in beds of the Jurassic and Triassic formations, 

 the remains of an equally great variety of these small mammalia have 

 been discovered; and from the examination of more than sixty speci- 

 mens, belonging to at least six distinct genera, Professor Marsh is of 

 the opinion that they represent a generalised type, from which the 

 more specialised marsupials and insectivora were developed. 



From the fact that very similar mammals occur both in Europe 

 and America at corresponding periods, and in beds which represent a 

 long succession of geological time, and that during the whole of this 

 time no fragments of any higher forms have been discovered, it seems 

 probable that both the northern continents (or the larger portion of 

 their area) were then inhabited by no other mammalia than these, 

 with perhaps other equally low types. It was, probably, not later 

 than the Jurassic age when some of these primitive marsupials were 

 able to enter Australia, where they have since remained almost com- 

 pletely isolated; and, being free from the competition of higher forms, 

 they have developed into the great variety of types we now behold 

 there. These occupy the place, and have to some extent acquired 

 the form and structure of distinct orders of the higher mammals — the 

 rodents, the insectivora, and the carnivora — while still preserving the 

 essential characteristics and lowly organisation of the marsupials. 

 At a much later period — probably in late Tertiary times — the ances- 

 tors of the various species of rats and mice which now abound in 

 Australia, and which, with the aerial bats, constitute its only forms 

 of placental mammals, entered the country from some of the adjacent 

 islands. For this purpose a land connection was not necessary, as 

 these small creatures might easily be conveyed among the branches 

 or in the crevices of trees uprooted by floods and carried down to the 

 sea, and then floated to a shore many miles distant. That no actual 

 land connection with, or very close approximation to, an Asiatic 

 island had occurred in recent times, is sufficiently proved by the fact 

 that no squirrel, pig, civet, or other widespread mammal of the Eastern 

 hemisphere has been able to reach the Australian continent. 



THE UISTKIBUTION OF BIRDS' 

 A. R. WALLACE 



These vary much in their powers of flight, and their capability of 

 traversing wide seas and oceans. Many swimming and wading birds 



' From A. R. Wallace, Darwinism (copyright iSgi). Used by special per- 

 mission of the publishers, The Macniillan Company. 



