ORGANIC EVOLUTION 



285 



has travelled a few hundred miles away from home should 

 be able to illustrate it by recalling the new form of animals 

 and plants met with in the new environments visited. 



3) Physiographic barriers. — We often find two closely 

 allied species in one locality inhabiting haunts that are just 

 a little different topographically. This is illustrated by two 

 of our common dragon flies, one of w^hich (Libellula semi- 

 fasciata) inhabits the small brooks and the other (L. pul- 



FiG. 171. A common pond inhabiting dragonfly (Libellula 

 pulchella). 



chella, fig. 171) the small ponds over a considerable part 

 of the United States. This matter will be abundantly 

 illustrated in Chapter VI under the subject of the adjust- 

 ment of organisms in place. 



4) By ecological differences. — Two species may live even 

 nearer to each other and yet dwell apart ; as in the case of 

 two species of squirrels of the same locality, one of which 

 burrows in the ground, while the other lives and nests in 

 trees. This sort of adjustment in place also will be studied 

 in Chapter VI. 



