Geographical Distribution. 207 



birds and insects may more or 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 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 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 



