Geographical Distribution. 207 
birds and insects may more or less easily have been 
able to fly from one to the other; while even non- 
flyirg 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 con- 
tinents 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 vege- 
tation which these large animals require, it is evident 
that scarcely any one species of cither 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 
