The most that observation gives us is the very imperfect suggestion of the track that such a movement would have left behind it, not unlike the scraps that boys litter along the road in a paper-chase. Similarly, if in the case of organic Evolution we deny all latent potentialities and preordained ends and throw the whole burden on accidental variations and natural selection; if we regard the whole process as no more intelligent or designed than that by which water seeks and finds its own level; yet as in the case of water we must perforce introduce "a gravitating tendency," so in the case of living organisms a "persisting" or "struggling tendency," as an hypothesis to give unity to our facts or to account for their uniformity. But these tendencies are as little matter of observation as the aforesaid latent potentialities or preordained ends. In fine, Evolution, whatever form it take, gets rid of theism and finality only by slipping into their place some tendency or indefinable power which it considers adequate to account for the facts to be explained. Let us now see if there be room in this philosophy for our argument from adaptability, and whether it will allow us to infer that because belief in theism and in future retribution are beliefs postulated by our higher moral aspirations, therefore they answer to reality more or less approximately; whether, in short, under certain conditions (specified in our last essay) the wish to believe may be a valid reason for believing. Now Evolution as a philosophy or explanatory hypothesis owes its popularity to its apparent simplicity. Wrapped in its wordy envelope, the notion as formulated by Spencer needs no subtilty of apprehension, but only a dictionary. Nor is the Darwinian theory of Natural Selection more difficult.