THE GEOGKAPHICAL DISTRIBUTION OF ANIMALS 373 



Other significant features of the distribution of animals. — 



A knowledge of the distribution of animals aids us very 

 much in determining the history of the earth. For example, 

 if we find a species of animal "having few or no facilities 

 for dispersal," on both sides of a very important barrier, 

 we feel pretty sure that there was a time in the history of 

 the earth when that particular barrier did not exist. Be- 

 cause, if it had always been in existence, the species of ani- 

 mal mentioned would not be found on both sides, since 

 the animal could not pass over the barrier. Hence a 

 knowledge of the distribution of animals often leads to the 

 disclosure of very profound changes in the topography of 

 the earth's surface. To illustrate, take one of the fresh- 

 water crayfishes of Great Britain. The same species is 

 found on the continent of Europe. Now, since this cray- 

 fish cannot pass across the salt water barrier of the English 

 channel, it would seem to indicate that this barrier did not 

 exist at some former time. In such a case, Great Britain 

 would have been simply a part of Europe. As a matter of 

 fact, such is the case, as has been proved by other and con- 

 clusive evidence. 



