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.' Given variability, 

 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 

 and the survival of the fittest a reality. 



DIRECT EVIDENCE FOR NATURAL SELEC- 

 TION. One of the interesting steps of prog- 

 ress since Darwin's day has been the 

 attempt to secure definite evidence of the 



