144 THE FIRST PRINCIPLES OF HEREDITY 



as regards Natural Selection. We may here say that 

 Natural Selection by means of discontinuous variations 

 has been accepted as an established fact, and Mendelism, 

 which shows the possibility of a recombination of parental 

 factors in a new form without transitional stages, has lent 

 much support to the Mutation theory of De Vries. To 

 what extent, and whether individual continuous variations 

 are a source of progressive evolution, is at present a moot 

 point. 



But our knowledge of variation has progressed further. 

 We are able to tell not only that organisms vary and how 

 they vary, but to what degree they vary ; in other words, 

 we have learnt to measure variability. 



Let us take, to follow R. H. Lock's statement of the 

 subject, measurements of any character e.g., the strength 

 of pull of certain men as recorded by Galton in his Natural 

 Inheritance in the following table : 



FIG. 61. TABLE OF STRENGTH OF PULL. (After Galton.) 



(From R. H. Lock, " Recent Progress in the Study of Variation, 

 Heredity, and Evolution.") 



We can arrange these measurements diagrammatically 

 in the following figure, where the horizontal scale gives us 

 the number of pounds pulled, and the vertical scale the 



