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 desired 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 itis 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 pessimisti: 
