CHAPTER III 



THE NATURE AND CAUSES OF VARIATION 



IN the last chapter we have endeavoured to explain 

 shortly the statistical method of dealing with variation. 

 It will be evident that we have considered such differ- 

 ences as occur between the individuals of a race only 

 in the mass. We have not attempted to distinguish 

 between the various kinds of differences, or the various 

 groups of causes which we believe to operate in pro- 

 ducing them. It is the object of the present chapter 

 to analyse the "sum total of observed differences," 

 and to discuss, as far as possible, the causes of 

 variation. 



In the first place we must make a sharp distinction, 

 at least theoretically, between an inborn variation and 

 a modification. Modifications, or " acquired characters," 

 are usually described as the abiding effects of external 

 conditions on the body of an organism ; whereas inborn 

 variations are due to changes in the germ plasm. But 

 it is necessary to proceed carefully in considering what 

 these definitions mean. For it is impossible to consider 

 any organism apart from the conditions under which 

 it lives apart, as we say, from its environment. The 

 inborn characters are expressed only under certain con- 

 ditions of environment, and acquired characters can 

 appear only when the organism possesses certain inborn 

 tendencies to respond to outside conditions. Thus in 

 a certain sense every character is both inborn and 

 acquired. 



But this does not affect the distinction. Given the 

 normal conditions of life within a particular race, the 

 organism would show only its inborn characters. Given 



