THE DISTRIBUTION OF SPECIES. 199 
mon with that of Africa, and it is, moreover, surrounded 
by transition districts which lead on the north to the 
holarctic and on the west to the Ethiop- 
ian. On the east the Indian realm is 
lost in the islands of Polynesia, which represents each 
one its own degree of transition and isolation. 
The Australian realm of Australia and its islands is 
more isolated than any of the others. It shows a sin- 
gular development of low or primitive 
types of vertebrate life, as though in the 
progress of evolution this continent had 
been left a whole geological age behind the.others. It 
is certain that, could the closely competing fauna of the 
holarctic or Indian realms have been able to invade 
Australia, the dominant mammals and birds of that ré- 
gion would not have been marsupials and parrots. Un- 
specialized types abound only where barriers have pre- 
vented competition. The larger the land area the greater 
the competition and the more specialized its character- 
istic forms. As part of this specialization is in the di- 
rection of hardiness, the species of the large experience 
are the more persistent and less easy of extermination. 
The rapid multiplication which certain holarctic animals 
and plants have shown when transported to the Aus- 
tralian realm, demonstrates what might have taken place 
if impassable barriers had not previously shut them out. 
Each of these great realms may be indefinitely sub- 
divided into provinces and sections, for there is no end 
to the possibility of analysis. No township or school 
district has exactly the same animals or plants as any 
other ; and, finally, in ultimate analysis, no two animals 
or plants are alike. Modification comes with the growth 
of each new individual, and steadily increases with the 
individual’s separation in time or space from the parent 
stock. Moreover, we observe apparent anomalies of 
Indian realm. 
Australian 
realm. 
