346 
GEOGEAPHICAL DISTRIBUTION. 
Chap. XI. 
CHAPTEE XL 
Geogbaphical Distribution. 
Present distribution cannot be accounted for by differences in phy¬ 
sical conditions — Importance of barriers — Affinity of the pro¬ 
ductions of the same continent — Centres of creation — Means 
of dispersal, by changes of climate and of the level of the land, 
and by occasional means — Dispersal during the Glacial period 
co-extensive with the world. 
In considering the distribution of organic beings over 
the face of the globe, the first great fact which strikes 
ns is, that neither the similarity nor the dissimilarity 
of the inhabitants of various regions can be accounted 
for by their climatal and other physical conditions. Of 
late, almost every author who has studied the subject 
has come to this conclusion. The case of America 
alone would almost suffice to prove its truth: for if we 
exclude the northern parts where the circumpolar land 
is almost continuous, all authors agree that one of the 
most fundamental divisions in geographical distribution 
is that between the New and Old Worlds; yet if we 
travel over the vast American continent, from the 
central parts of the United States to its extreme 
southern point, we meet with the most diversified con¬ 
ditions ; the most humid districts, arid deserts, lofty 
^mountains, grassy plains, forests, marshes, lakes, and 
great rivers, under almost every temperature. There is 
hardly a climate or condition in the Old World which 
cannot be paralleled in the New—at least as closely 
as the same species generally require; for it is a most 
rare case to find a group of organisms confined to any 
small spot, having conditions peculiar in only a slight 
