SELECTION 161 



theory of natural selection has the marks of 

 a good theory it works well as an interpre- 

 tative formula in the most varied cases, it 

 has proved itself a useful instrument of 

 research, and it has even been made the 

 basis of successful prediction. Darwin him- 

 self was under no misapprehension as to the 

 logical position of his theory that its 

 strength was in its interpretative 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 en- 

 tirely on general considerations (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.' 5 



a high rate of "increase/ the struggle for 

 existence, the web of life, the observed fact 

 that most living creatures die young it 

 seems to most naturalists to follow that 

 natural selection is indeed a vera causa 

 ^the suryival oLJthg fittest a reality* > 

 IRECT EVIDENCE FOR KATURAL SELEC- 

 TION. One of the interesting steps of prog- 

 ress since Darwin's day has been the 

 attempt to secure definite evidence of the 



