NATURAL SELECTION 121 



the facts to which he appealed. Before deciding on 

 the merits of natural selection this evidence must he 

 examined. 



The Measurement of Variation 



If we measure, or weigh, or classify any character 

 shown hy the individuals of a po^mlation, we find 

 much variahilitv some of which we ascrihe to the 

 varied experiences that the individuals have encount- 

 ered in the course of their lives, i.e., to their environ- 

 ment, hut we also recognize that some of the differ- 

 ences may he due to individuals having different 

 inheritances. A few familiar examples will help to 

 hring out this contrast. 



If the leaves of a tree are arranged according to 

 size {fig. 53), we find a continuous series, hut there 



Fig. 53. — Series of leaves of a tree arranged according 

 to size. (After DeVries.) 



are more leaves of medium size than extremes. If a 

 lot of beans be sorted out according to their weights, 

 and those between certain weights put into cylinders, 

 the cylinders, when arranged according to the size 



