138 THE THEORY OF EVOLUTION 



probable, however, that a state of equilibrium has 

 been reached after the readjustments made neces- 

 sary by the last geographical and climatic changes. 

 On the other hand, a slow movement may actually 

 be in progress, as is suggested by the geographical 

 races of the Virginia Deer from Mexico to Guiana. 

 Central America contains a considerable number of 

 Sonoran mammals, which have not yet passed the 

 Isthmus, but might perhaps eventually have done 

 so, had not the construction of the canal intervened. 



The elevation of Central America and the Isthmus 

 cut off the waters of the Caribbean Sea from those 

 of the Pacific and the separation led to divergent 

 development of marine organisms in the divided 

 waters. A careful comparative study has been made 

 of the sea-fishes on the two sides of the Isthmus, with 

 the result that nearly all the species are different, 

 while the genera and families are, for the most part, 

 the same. The amount of difference is what might 

 have been expected d priori on the evolutionary 

 hypothesis and, indeed, the ichthyologists had ap- 

 proximated the true date of the separation before 

 the geological evidence had been found. 



There is no time to go into the highly complex 

 history of the vast aggregation of land in the Old 

 World, further than to point out that as late as the 

 Pleistocene epoch, when primitive men were already 

 living in Europe, that continent underwent a re- 

 markable series of climatic and geographical changes. 

 At the time of greatest elevation, the Adriatic and 



