" And yet I believe I have taken the theory almost at its best— almost in its most testable form. One might say that it “almost predicts” a great variety of forms of life.283 In other fields, its predictive or explanatory power is still more disappointing. Take “adaptation”. At first sight natural selection appears to explain it, and in a way it does; but hardly in a scientific way. To say that a species now living is adapted to its environment is, in fact, almost tautological. Indeed we use the terms “adaptation” and “selection” in such a way that we can say that, if the species were not adapted, it would have been eliminated by natural selection. Similarly, if a species has been eliminated it must have been ill adapted to the conditions. Adaptation or fitness is defined by modern evolutionists as survival value, and can be measured by actual success in survival: there is hardly any possibility of testing a theory as feeble as this.
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Now to the degree that Darwinism creates the same impression, it is not so very much better than the theistic view of adaptation; it is, therefore important to show that Darwinism is not a scientific theory, but metaphysical. But its value for science as a metaphysical research programme is very great, especially if it is admitted that it may be criticized, and improved upon. "

― Karl Popper , Unended Quest: An Intellectual Autobiography