Origin of Species 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 va