30 



NATURAL HISTORY 



are necessarily limited in their distribution. A second means of 

 repression lies in the poor adaptability of organisms to new localities 

 which they have invaded. A round peg will not fit in a square hole, 

 nor a square peg in a round hole, but if the peg consists of a plastic 

 material it will adapt itself. The normal habitat for a species is the 

 place where the organism is most nearly in physiological equilibrium, 

 the geographic range being determined by the fluctuation of a factor, 

 or factors, which are necessary for the life of a species. 



Barriers 



Each species widens its range of distribution as far as possible and 

 tries to overcome obstacles which nature has put in its way. These 

 obstacles may be chemico-physical, geographical, or biological bar- 

 riers. 



In general chemico-physical barriers are climatic in nature, such 

 as unfavorable conditions of moisture, soil, or temperature. Soil 

 deficiencies, salinity, the presence or absence of light, or character 

 of the surrounding medium might also be mentioned. These climatic 



Friislur 



Why might such a mountain barrier restrict the distribution of certain plants 



and animals .3 



