Darwin s Artificial and Natural Selection 157 



when carefully looked into encountered numerous difficulties, 

 which were formidable, for the reason that we were unsuc- 

 cessful in tracing out the actual details of the individual pro- 

 cess, and, therefore, in fixing the phenomenon as it actually 

 occurred. We can state in no single case how great a varia- 

 tion must be to have selective value, nor how frequently it 

 must occur to acquire stability. We do not know when and 

 whether a de_sired_ useful variation really occurs, nor on what 

 its appearance depends; and we have no means of ascertain- 

 ing the space of time required for the fulfilment of the'selec- 

 tive processes of nature, and hence cannot calculate the 

 exact number of such processes that do and can take place at 

 the same time in the same species. Yet all this is necessary 

 if we wish to follow out the precise details of a given case. 



" But perhaps the most discouraging circumstance of all 

 is, that we can assert in scarcely a single actual instance in 

 nature whether an observed variation is useful or not — a 

 drawback that I distinctly emphasized some time ago. Nor 

 is there much hope of betterment in this respect, for think 

 how impossible it would be for us to observe all the individ- 

 uals of a species in all their acts of life, be their habitat ever 

 so limited — and to observe all this with a precision enabling 

 us to say that this or that variation possessed selective value, 

 that is, was a decisive factor in determining the existence of 

 the species." 



" And thus it is everywhere. Even in the most indubitable 

 cases of adaptation as, for instance, in that of the striking pro- 

 tective coloring of many butterflies, the sole ground of infer- 

 ence that the species on the whole is adequately adapted to 

 its conditions of life, is the simple fact that the species is, 

 to all appearances, preserved undiminished, but the inference 

 is not at all permissible that just this protective coloring has 

 selective value for the species, that is, if it were lacking, 

 the species would necessarily have perished." 



Few opponents of Darwinism could give a more pessimistic 



