Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions 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 -- Alternate Glacial periods in the North and
South.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be wholly accounted for by 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 arctic and northern temperate parts, 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 conditions;
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 so closely as the same species generally require.
Pages:
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591