Read this: Later chapters provide evidence that evolution has occurred, supporting the idea of branching, adaptive evolution without directly proving that selection is the mechanism. Darwin presents supporting facts drawn from many disciplines, showing that his theory could explain a myriad of observations from many fields of natural history that were inexplicable under the alternate concept that species had been individually created. The structure of Darwin's argument showed the influence of John Herschel, whose philosophy of science maintained that a mechanism could be called a vera causa (true cause) if three things could be demonstrated: its existence in nature, its ability to produce the effects of interest, and its ability to explain a wide range of observations.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What are the three things John Herschel maintains need to be shown to allow for a mechanism be called a true cause?
its existence in nature, its ability to produce the effects of interest, and its ability to explain a wide range of observations