22 ANIMAL LIFE AND SOCIAL GROWTH 



mean that there will be a similarity in the animals 

 living in those habitats. There are historical 

 factors to be considered which concern the op- 

 portunities animals have had to occupy the par- 

 ticular space. Obviously insurmountable bar- 

 riers of water or mountains or deserts may block 

 the path of migration of animals well fitted to 

 occupy a given habitat, or more subtle barriers 

 of climate may accomplish the same result. Fur- 

 ther, suitable species may have arisen too recently 

 to have had time to reach all the habitats they 

 are fitted to occupy to advantage, or on the other 

 hand, other species may be dying out from lack 

 of racial vigor sufficient to maintain themselves 

 even in territory in which they formerly lived in 

 strength. 



In some instances mere chance occupancy by 

 one species may prevent another later arrival 

 with similar habitat requirements from being able 

 to obtain a foothold. An example of this sort is 

 furnished by the distribution of crayfishes in 

 Pennsylvania. Two species with similar ecolog- 

 ical requirements formerly occupied different 

 river systems which later joined into one. At 

 the old water shed where the ranges of the two 

 species now come together without an environ- 

 mental obstacle there is only an exceedingly 

 narrow zone of overlap. The niches required 



