And these variations are, after all, due to causes, but to unknown causes; spontaneous variation being, in fact, but another name for variation due to causes which we know nothing about, but in no possible sense a _cause of variation_. So that when we come to put clearly before our minds exactly what the sentence we are considering amounts to, it comes to this: that it is impossible to attribute the innumerable structures which are so well adapted to the habits of life of each species to _unknown causes_. “I can no more believe in _this_,” continues Mr. Darwin, “than that the well-adapted form of a race-horse or greyhound, which, before the principle of selection by man was well understood, excited so much surprise in the minds of the older naturalists, can _thus_ be explained” (“Natural Selection,” p. 171, ed. 1876). Or, in other words, “I can no more believe that the well-adapted structures of species are due to unknown causes, than I can believe that the well-adapted form of a race-horse can be explained by being attributed to unknown causes.” I have puzzled over this paragraph for several hours with the sincerest desire to get at the precise idea which underlies it, but the more I have studied it the more convinced I am that it does not contain, or at any rate convey, any clear or definite idea at all. If I thought it was a mere slip, I should not call attention to it; this book will probably have slips enough of its own without introducing those of a great man unnecessarily; but I submit that it is necessary to call attention to it here, inasmuch as it is impossible to believe that after years of reflection upon his subject, Mr. Darwin should have written as above, especially in such a place, if his mind was really clear about his own position. Immediately after the admission of a certain amount of miscalculation, there comes a more or less exculpatory sentence which sounds so right that ninety-nine people out of a hundred would walk through it, unless led by some exigency of their own position to examine it closely but which yet upon examination proves to be as nearly meaningless as a sentence can be.