r less easily have been able to fly from
one to the other; while even non-flying animals and plants may often
have been transported by floating ice or timber, wind or water currents,
and sundry other means of dispersal. Again, there is the important
influence of climate to be taken into account. We know from geological
evidence that in the course of geological time the self-same continents
have been submitted to enormous changes of temperature--varying in fact
from polar cold to almost tropical heat; and as it is manifestly
impossible that forms of life suited to one of these climates could have
survived during the other, we can here perceive a further and most
potent cause interfering with the test of geographical distribution as
indiscriminately applied in all cases. When the elephant and
hippopotamus were flourishing in England amid the luxuriant vegetation
which these large animals require, it is evident that scarcely any one
species of either the fauna or the flora of this country can have been
the same as it was when its African climate gave place to that of
Greenland. Therefore, as Mr. Wallace observes, "If glacial epochs in
temperate lands and mild climates near the poles have, as now believed
by men of eminence, occurred several times over in the past history of
the earth, the effects of such great and repeated changes both on
migration, modification, and extinction of species, must have been of
overwhelming importance--of more importance perhaps than even the
geological changes of sea and land."

But although for these, and certain other less important reasons which I
need not wait to detail, we must conclude that the evidence from
geographical distribution is not to be regarded as a crucial test
between the rival theories of creation and evolution in all cases
indiscriminately, I must next remark that it is undoubtedly one of the
strongest lines of evidence which we possess. When we once remember
that, according to the general theory of evolution itself, the present
geographical distribution of plants and animals is "the visible outcome
or residual product of the whole past history of the earth," and,
therefore, that of the conditions determining the characters of life
inhabiting this and that particular area continuity or discontinuity
with other areas is but one,--when we remember this, we find that no
further reservation has to be made: all the facts of geographical
distribution speak with one consent in f