SELECTION 161 



with reference to these variations, then the 

 possessors of the fitter variations are bound 

 to be favoured with longer life and larger 

 families with survival, in short. If this is 

 kept up consistently, then new adaptations, 

 and, probably with the help of some form of 

 isolation, new species, will arise. 



THE CASE FOR NATURAL SELECTION. The 

 theory of natural selection has the marks of 

 a good theory it works well as an interpreta- 

 tive formula in the most varied cases, it has 

 proved itself a useful instrument of research, 

 and it has even been made the basis of success- 

 ful prediction. Darwin himself was under no 

 misapprehension as to the logical position of 

 his theory that its strength was in its in- 

 terpretative value, not in its direct evidence. 

 In a letter to Bentham in 1863, he writes : 

 " The belief in natural selection must at 

 present be grounded entirely on general con- 

 siderations (1) on its being a vera causa, 

 from the struggle for existence and the certain 

 geological fact that species do somehow 

 change; (2) from the analogy of change under 

 domestication by man's selection; (3) and 

 chiefly from this view connecting under an 

 intelligible point of view a host of facts." 

 Given variability, a high rate of increase, the 

 struggle for existence, the web of life, the 





