VARIATION IN GENERAL 15 



the molecule of chemical compounds. Such elementary charac- 

 ters are supposed not to be variable except as they effect dif- 

 ferent combinations among themselves. 



SECTION VI THE UNIT OF VARIABILITY 



The unit of variation is therefore not the individual but the 

 racial characters that constitute the particular group, and that 

 run down the line of descent like the strands of a rope and out 

 of which individuals are made up, some with one combination, 

 others with another, after the fashion of threads in a fabric, 

 forming patterns here and there, now of one design now of 

 another, as they wander apparently aimlessly here and there. 1 



It is evident, however, that the actual basis of character devia- 

 tion is sometimes exceedingly complex. Milk secretion, for 

 example, while limited to certain animals and confined to the 

 female sex, is properly recognized as a distinct character ; yet 

 its successful functioning depends upon a variety of considera- 

 tions, the general health of the body, the nervous tempera- 

 ture of the individual, the power of the stomach to provide large 

 quantities of prepared food, the ability of the kidneys to do 

 their work, and the power of every organ in the body to dis- 

 charge its function successfully and fully under heavy strains. 

 All these are as important to successful milk production as 

 are large and active milk glands, and an accident at any point 

 will cause deviation in milk yield either as to quantity or quality 

 or both. 



Deviation in color, on the other hand, may be due to presence 

 or absence of pigment, which may be regarded as a chemical 

 substance secreted at a single point. In this and in similar 

 cases the actual basis of deviation is simple and readily detected. 

 From this it will appear that the ultimate seat of variation, 

 whose fluctuations are responsible for character deviations, may 

 be exceedingly difficult if not impossible to identify. 



1 The unit of variability must not be confused with the unit of selection : the 

 latter, of course, is the individual. We cannot separate his characters, but must 

 take him as he is, for better or for worse ; but we must do so fully realizing that 

 each of his separate characters has an identity of its own, so that the unit of vari- 

 ability is far within the necessary unit of selection. 



