CHAPTER FIFTY-THREE 



Environ- 

 mental and 

 historic 

 factors 



How the 

 past affects 

 the present 



THE GEOGRAPHICAL DISTRIBUTION OF LIFE 



I. IT is well known to all that the various forms of 

 animal and plant life are not distributed uniformly over 

 the earth's surface. When we seek to determine why 

 the range of different species is limited, we find that the 

 factors involved may be sorted out into two great 

 groups, which may be termed the environmental and 

 historic. The environmental factors are those which 

 determine the possibility of existence in a given locality. 

 Thus a fish cannot live on land ; a tropical bird, trans- 

 ported to the arctic regions, would probably die in half 

 an hour. These are the simplest cases, depending on 

 physical conditions of the most obvious sort, but many 

 other factors also must be classed as environmental. 

 Thus the chestnut tree cannot exist in regions, invaded 

 by the chestnut-blight fungus ; mice perish in the 

 presence of a sufficient number of cats. In these ex- 

 amples the death-dealing causes may be directly ob- 

 served, but many others escape our notice. Causes of 

 death or failure to reproduce (which biologically comes 

 to the same thing) frequently result in extermination 

 only after a long period, and then the process is too slow 

 to be conspicuous to the casual observer. 



2. Historic factors have to do, not with the effects of 

 the environment, but with the ability of the organism to 

 reach given localities. The question is not, Can you 

 live here ? but, Were you able to get here ? Humming 

 birds would presumably flourish in tropical Asia and 

 Africa, but they have never been able to cross the 

 Atlantic or Pacific. Many European insects and weeds 

 prosper exceedingly in America after being accidentally 

 brought over by man, but in pre-Columbian times they 



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