And here I think it needful, although not for all readers, but for the great majority, to lay down as distinctly as I can the rules of evidence which necessarily govern our beliefs. I do so because, in reading the works of many of the modern scientists who have espoused the Darwinian doctrine of evolution, I find that the rules of evidence are but little observed. There is a very great, often an astonishingly great, accumulation of facts, or of assumed facts. It is impossible not to be impressed by the learning, the industry, and the range of these writers. Nor would I in the least impugn their candor, or question their accuracy as witnesses of facts, which I am not competent to dispute if I were disposed to do so. But there is one thing for which I may suppose myself competent. I have through a long life been accustomed to form conclusions upon facts; and this is what every person does and must do who is asked to accept a new theory or hypothesis of any kind upon any subject. Most of our beliefs depend upon what is called circumstantial evidence. There are very few propositions which address themselves to our belief upon one direct and isolated proof. We may class most of the perceptions of our senses among the simple and unrelated proofs which we accept without hesitation, although there is more or less of an unconscious and instantaneous process of reasoning, through which we pass before the evidence of our senses is accepted and acted upon. Then there are truths to which we yield an instant assent, because they prove themselves, as is the case with the mathematical or geometrical problems, as soon as we perceive the connection in the steps of the demonstration. Besides these, there are many propositions which, although they involve moral reasoning, have become axioms about which we do not care to inquire, but which we assume to have been so repeatedly and firmly established that it would be a waste of time to go over the ground again whenever they come up. But there is a very large class of propositions which address themselves to our belief, which do not depend on a single perception through our senses, and are not isolated facts, and are not demonstrable by mathematical truth, and are not axioms accepted because they were proved long ago, and have by general consent been adopted into the common stock of ideas. The class of beliefs with which the rules of circumstantial evidence are concerned are those where the truth of the proposition, or hypothesis, is a deduction from many distinct facts, but the coexistence of which facts leads to the inevitable conclusion that the proposition or hypothesis is true. We can not tell why it is that moral conviction is forced upon us by the coexistence of certain facts and their tendency to establish a certain conclusion. All we know is, that our minds are so constituted that we can not resist the force of circumstantial evidence if we suffer our faculties to act as reason has taught them.