Weismann is not, however, of this opinion, and believes that the present depression is only transient, because it is only a reaction against a theory that had been exalted to the highest pinnacle. He thinks that the principle of selection is not overestimated, but that naturalists imagined too quickly that they understood its workings. “On the contrary, the deeper they penetrated into its workings the clearer it appeared that something was lacking, that the action of the principle, though upon the whole clear and representable, yet when carefully looked into encountered numerous difficulties, which were formidable, for the reason that we were unsuccessful in tracing out the actual details of the individual process, and, therefore, in _fixing_ the phenomenon as it actually occurred. We can state in no single case how great a variation 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 ascertaining the space of time required for the fulfilment of the selective 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 individuals 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 protective coloring of many butterflies, the sole ground of inference 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.”