EVOLUTION AS A NATURAL PROCESS 121 



which might not be represented among one thousand 

 people. 



It is not possible to explain why variation should 

 follow this or any other mathematical law without 

 entering into an unduly extensive discussion of the 

 laws of error. The mathematicians themselves tell 

 us in general terms that the observations they describe 

 so simply by their formulae follow as the result of 

 so-called chance, by which they mean that the combined 

 operation of numerous, diverse, and uncorrelated factors 

 brings about this result, and not, of course, that there 

 is such a thing as an uncaused event or phenomenon. 



Whenever any extensive series of like organisms has 

 been studied with reference to the variations of a 

 particular character, the variations group themselves so 

 as to be described by identical or similar curves of error. 

 It is certainly significant that this is true for such 

 diverse characters, cited at random from the lists of 

 the literature, as the number of ray-flowers of white 

 daisies, the number of ribs of beech leaves, and of the 

 bands upon the capsules of poppies, for the shades of 

 color of human eyes, for the number of spines on the 

 backs of shrimps, and for the number of days that 

 caterpillars feed before they turn into pupae. 



To summarize the foregoing facts, we have learned 

 that variation is universal throughout the living world, 

 and that the primary factors causing organic dif- 

 ference the counterparts of human ingenuity in 

 the case of dead mechanisms are the natural in- 

 fluences of the environment, of organic physiological 

 activity, and of congenital inheritance. These factors 

 are accorded different values in the evolution of new 



