En ausencia de pruebas, mejor creer, concluye el autor en al párrafo centésimo nonagésimo noveno de El Origen de las Especies

  199.   It is very difficult to decide how far changed conditions, such as of climate, food, etc., have acted in a definite manner. There is reason to believe that in the course of time the effects have been greater than can be proved by clear evidence. But we may safely conclude that the innumerable complex co-adaptations of structure, which we see throughout nature between various organic beings, cannot be attributed simply to such action. In the following cases…