INTANGIBLE BARRIERS 475 



or to those requiring considerable amounts of moisture. 

 Large bodies of water set an effectual limit to the scat- 

 tering of large terrestrial animals, and rivers, streams, 

 and lakes are as impassable in the case of many smaller 

 species. On the other hand, it is equally manifest that 

 the movements of aquatic forms are checked by inter- 

 vening tracts of land. Prairies and meadows, forests 

 and brushy country, rocky outcrops, differences in the 

 character of the soil and many other local features act as 

 additional barriers to the wide dispersal of many species. 



Intangible Barriers. — Adult animals may migrate 

 or be transported into a new territory that suits their in- 

 dividual needs admirably, and yet be unable to exist for 

 any length of time on account of the lack of suitable 

 shelter from the attacks of enemies. Birds, for example, 

 may be unable to rear their offspring owing to the absence 

 of trees in which to build safe homes. Climatic condi- 

 tions may also render a locality unsuitable as a habita- 

 tion for numerous animals, yet even here a few are often 

 able to exist if other conditions are favorable. The 

 camel delights in a warm country, yet it exists in Mon- 

 golia, and tigers, certain apes, and the elephant, which 

 we may think of as only tropical animals, are not infre- 

 quently found in regions where the winters are com- 

 paratively severe. But all of these intangible barriers — 

 enemies, lack of shelter or refuges, and climatic conditions 

 — are of less importance than food. An animal or a 

 species, without food but in an otherwise favorable situa- 

 tion, becomes extinct; with sufficient nourishment each 

 may persist, even though conditions are severe. 



Laws and Results of Distribution. — It now follows 

 from the facts noted in the preceding paragraph that (a) 

 every species of animal is found in every quarter of the 

 globe unless it has been hindered by some kind of bar- 

 rier. If it is able to penetrate this barrier (6) it persists 

 where the conditions of life are favorable. If it does 

 continue and the surrounding conditions remain prac- 



