AGENCIES WHICH GOVERN THE DISTRIBU- 

 TION OF LIFE 



A. BEAZIEK HOWELL 

 Pasadexa, Calif. 



The problems presented by the distribution of plants 

 and animals is a fertile field for investigation. These 

 problems are essentially ecological in character, for 

 often, perhaps always, the range of a species or genus 

 is dependent upon a number of diverse environmental 

 factors, some of which are readily apparent, while others 

 are obscure ; but always they merit careful study. 



In investigating and mapping the ranges of living 

 organisms and in following the evolutional tendencies of 

 species in so far as we are able, environment and its in- 

 fluences are of the greatest moment, especially from an 

 ecological standpoint. Botanical subjects may usually 

 be allocated in relation to their surroundings with con- 

 isiderably greater ease than can active forms of life, for 

 the former are acted upon only by the agencies to be 

 found in one spot, while the latter may experience not 

 only all the influences operative over several square 

 miles, more or less, of diversified territory, but, in the 

 case of a migratory bird or mammal, will be subject dur- 

 ing a part of the year to environmental factors of which 

 we may know nothing. Whether a species is common or 

 rare in a certain area depends upon its rate of reproduc- 

 tion, which is usually entirely adequate unless new and 

 disturbing influences have been introduced; upon the 

 number of favorable or unfavorable conditions which it 

 encounters, the amount of competition with which it has 

 to contend, and its phylogenetic characters, as to whether 

 it be of a plastic type or one which is senescent and over- 

 specialized : all of which may be summed up in the phrase 

 ''adaptability to its habitat." 



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