Our position, then, is this. On the foundation of the previous chapters, I will henceforth assume that we all accept organic evolution as a fact, without requiring any of the accessory evidence which is gained by independent proof of natural selection as a method. But in making this assumption--namely, that we are all now firmly persuaded of the fact of evolution--I do not imagine that such is really the case. I make the assumption for the purposes of systematic exposition, and in order that different parts of the subject may be kept distinct. I confess it does appear to me remarkable that there should still be a doubt in any educated mind touching the general fact of evolution; while it becomes to me unaccountable that such should be the case with a few still living men of science, who cannot be accused of being ignorant of the evidences which have now been accumulated. But in whatever measure we may severally have been convinced--or remained unconvinced--on this matter, for the purposes of exposition I must hereafter assume that we are all agreed to the extent of regarding the process of evolution as, at least, sufficiently probable to justify enquiry touching its causes on supposition of its truth. Now, the causes of evolution have been set forth in a variety of different hypotheses, only the chief of which need be mentioned here. Historically speaking the first of these was that which was put forward by Erasmus Darwin, Lamarck, and Herbert Spencer. It consists in putting together the following facts and inferences.