112 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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 DISTRIBUTION 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 
tFrom A. R. Wallace, Darwinism (copyright 1891). Used by special per- 
mission of the publishers, The Macmillan Company. 
