CHAPTER II 



VARIATION 



If it were an absolute fact that like begets like, instead 

 of a general tendency, it is apparent that there would 

 be no science of heredity at all. All the individuals 

 of a race would be identical in t^^e, and heredity would 

 offer no problem except that of accounting for this 

 identity by some satisfactory theory. 



But it is a trite observation, wliich we need not 

 hnger to enlarge upon, that no two things in nature 

 are ever exactly alike. Before entering into the ques- 

 tion of how or to what extent these differences are 

 inherited, it is necessary that we should gain some 

 insight into their nature and causes. 



In this connection, our first task must be to find 

 some method of describing accurately and briefly such 

 variations as are found. In certain cases this is a very 

 easy matter. Suppose, for instance, that we -ft-ished to 

 describe the variation in colour in a certain breed of 

 horses. All that we have to do is to count the number 

 of bays, blacks, greys, &c., occurring in a group of 

 individuals taken at random, and to state the numbers 

 of the respective colours. In tliis case we are deahng 

 with material which is at once separable into definite 

 natural groups. The colour does not vary by minute 

 stages from black to wliite, but the variation is, as we 

 say, discontinucnis. 



With regard to a great many characters, however, 

 variation is continuous. As regards statiure, for in- 

 stance, men are not separable into so many definite 

 types, but there is a fairly regular gradation in size, 

 from the giant to the dwarf. How are we to measure 

 and represent variation of this kind ? 



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