1 84 EVOLUTION OF TO-DAY. 



be crossed ? The only method of explanation is to 

 show either that the barriers in question do not 

 act as rigidly as at first sight seems, or to show that 

 they have not always existed. One series of dif- 

 ficulties disappears when we remember that the 

 physical conditions of the land have not always 

 been what they are now. The Pacific Ocean and 

 the Gulf of Mexico are now separated by a bar- 

 rier impassable to fishes ; but this has not always 

 been the case. We have only to assume the depres- 

 sion of the land a few hundred feet from the 

 isthmus, and we should have the two oceans united. 

 Here is a ready explanation of the first difficulty 

 mentioned. Africa was once connected with Eu- 

 rope, though now it is practically separated from 

 it. Similarly Australia was formerly connected to 

 Asia. Many of our mountain ranges are of com- 

 paratively recent origin, and although now they 

 form effectual barriers, they have not always done 

 so. If, therefore, a species is very old we might 

 find representatives of it on either side of such a 

 recently created barrier, and not unfrequently might 

 we find species of this same genus thus separated. 



To explain the class of facts illustrated by the 

 second of the above examples, naturalists have 

 studied with great care the means of dispersal of 

 animals, with the result of proving that the so-called 

 barriers are not always so rigid in their action as 

 at first appears. It would seem that fresh-water 

 animals would be very limited in their powers of 

 migration; would be confined to a single river or 

 system of rivers. For, unable to live in the air or in 



