Evolution, as a scientific theory, is a description of the process by which the totality of things has come to be what it is. The method employed is that of science, and proceeds upon the assumption of the uniformity of Nature and the universality of the law of causation. The existence of a thing is proof that the conditions necessary to produce it preceded it. Thus from what is we infer with certainty what has been: the occurrence of Z is proof that Y preceded, and so from Y we can infer X, and so on, to the beginning of the alphabet. Eventually, that is, we are carried back, in theory at least, to an initial arrangement of things which not only gave birth to the actual order of evolution, but was such that no other order of events could have followed from it. Were it possible, in fact, to get back to this original collocation of causes and to formulate it, the formula would explain the universe as it is and has been, the totality of things. Unfortunately the formula, though it would explain everything else, would not explain itself, and would therefore, so far, fail to explain anything. Or, to put it in other words, though certain causes, collocated in the proper way, would, on this view of evolution, explain everything which ensued from that collocation, we should still want to know why the causes were collocated in that particular way rather than in any other. To say that that collocation was not the outcome of a previous collocation is really to say that there was originally no antecedent necessity why this or any other order of evolution should take place at all; that Z hangs on Y, Y on X ... and A on nothing at all; that the formula which is to render all things intelligible is itself unmeaning. Or, if we say that things had no beginning--matter and force being indestructible--then there is no initial collocation, that is to say, no formula, even in theory, to explain all things: we cannot even imagine the process of evolution to be intelligible.