CHAPTER IX 



EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



It was long ago noted, and the most casual observer cannot 

 fail to discover, that individuals of the same species vary greatly 

 according to their environment, meaning by that term all the 

 external conditions of life, such as climate, food, friends, enemies, 

 and all those outside influences, favorable or unfavorable, among 

 which the individual finds itself born, and with which it must 

 live upon the best terms possible if it would live at all. That 

 these external agencies exert a direct effect upon living matter 

 is beyond question, and it remains to give attention to the nature 

 and extent of this influence as a partial answer to the question 

 we would solve, the dependence of organized living matter 

 upon the external world for the nature and range of its activities. 

 Anything we may learn upon this point will be a contribution to 

 the stock of knowledge out of which we shall one day determine 

 all the causes of variation. 



Without a doubt the great bulk of variability is due to 

 causes internal to the organism, mainly in the form of inherited 

 tendencies. Pearson, after exhaustive statistical investigations, 

 remarks, " The individual contains within itself, owing to a bath- 

 mic law of growth, a variability which is itself quite sensible, 

 being 80 or 90 per cent of the variability of the race." 1 



Even then, however, these internal influences are dependent 

 upon outside conditions for their opportunity. A born giant 

 must have food in abundance, but no amount of food would 

 make a giant out of a dwarf. Nor will it avail to awaken, late 

 in life, forces that once might have been active. Some dwarfs 

 are therefore born and others are produced by insufficient food. 



The external conditions of life affect variability in four dis- 

 tinctly different ways : (i) through natural selection, influencing 



1 Pearson, Grammar of Science, p. 473. 

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