272 Evolution and Adaptation 



of men having a mode of 5 feet 9 inches, and the 6-foot men 

 were selected in each generation, then in six generations this 

 type would be permanently established, and this change 

 could be effected in two hundred years. 1 



Thus we have exact data as to what will happen on the 

 average when blended, fluctuating variations are selected. 

 Important as such data must always be to give us accurate 

 information as to what will occur if things are left to 

 " chance " variations, yet- if it should prove true that evolu- 

 tion has not been the outcome of chance, then the method is 

 entirely useless to determine how evolution has occurred. 



More important than a knowledge of what, according to 

 the theory of chances, fluctuating variations will do, will be 

 information that would tell us what changes will take place 

 in each individual. In this field we may hope to obtain data 

 no less quantitative than those of chance variations, but of a 

 different kind. A study of some of the results of discon- 

 tinuous variation will show my meaning more clearly. 



Discontinuous Variation 



Galton, in his book on " Natural Inheritance," points out 

 that " the theory of natural selection might dispense with 

 a restriction for which it is difficult to see either the need 

 or the justification, namely, that the course of evolution al- 

 ways proceeds by steps that are severally minute and that 

 become effective only through accumulation." An apparent 

 reason, it is suggested, for this common belief " is founded on 

 the fact that whenever search is made for intermediate forms 

 between widely divergent varieties, whether they are of plants 

 or of animals, of weapons or utensils, of customs, religion, or 

 language, or of any other product of evolution, a long and 

 orderly series can usually be made out, each member of which 

 differs in an almost imperceptible degree from the adjacent 



1 Quoted from Pearson's " Grammar of Science." 



