6 THE MEASUREMENT OF VARIATION. 



there was never any constancy, every animal varying 

 in respect of each of the measurements made. This 

 is a point of fundamental importance, which cannot be 

 too thoroughly grasped. Every organism varies in re- 

 spect of all its characters, whatever be their nature. 

 The amount of this variation differs greatly, as these 

 results well show, but it is always present in a greater 

 or less degree. Another fact which this diagram brings 

 out very clearly is the comparative independence of 

 these measurements. Because the body of one animal 

 is longer than another, it by no means necessarily fol- 

 lows that the head or tail is longer also. A superficial 

 glance at this diagram might, indeed, lead one to sup- 

 pose that the various parts of the body were absolutely 

 independent of each other. But this we know not to 

 be the case. Between most parts and organs there is a 

 greater or less degree of correlation, so that, on an aver- 

 age, animals with a longer body may have a longer head 

 and longer tail than animals with a shorter body. A 

 careful examination of the diagram will show that, on 

 the whole, though with numerous exceptions, the curves 

 for head, tail, and foot do slope very slightly upwards 

 from left to right, though nothing like as much as the 

 curve for body lengths. Some degree of correlation is 

 therefore present, though it is only slight. We know 

 that frequently it may be very great indeed, as for in- 

 stance between the two fore limbs or two hind limbs of 

 a quadruped, and very considerable between a fore and 

 a hind limb; but into this question we must not enter 

 now. 



Almost innumerable diagrams of a similar nature to 

 the above might be given, but this is scarcely neces- 



