I have nothing to say to any “Philosophy of Evolution.”  Attempts to construct such a philosophy may be as useful, nay, even as admirable, as was the attempt of Descartes to get at a theory of the universe by the same a priori road; but, in my judgment, they are as premature.  Nor, for this purpose, have I to do with any theory of the “Origin of Species,” much as I value that which is known as the Darwinian theory.  That the doctrine of natural selection presupposes evolution is quite true; but it is not true that evolution necessarily implies natural selection.  In fact, evolution might conceivably have taken place without the development of groups possessing the characters of species.

For me, the doctrine of evolution is no speculation, but a generalisation of certain facts, which may be observed by any one who will take the necessary trouble.  These facts are those which are classed by biologists under the heads of Embryology and of Palaeontology.  Embryology proves that every higher form of individual life becomes what it is by a process of gradual differentiation from an extremely low form; palaeontology proves, in some cases, and renders probable in all, that the oldest types of a group are the lowest; and that they have been followed by a gradual succession of more and more differentiated forms.  It is simply a fact, that evolution of the individual animal and plant is taking place, as a natural process, in millions and millions of cases every day; it is a fact, that the species which have succeeded one another in the past, do, in many cases, present just those morphological relations, which they must possess, if they had proceeded, one from the other, by an analogous process of evolution.

The alternative presented, therefore, is:  either the forms of one and the same type—­say, e.g., that of the Horse tribe[13]—­arose successively, but independently of one another, at intervals, during myriads of years; or, the later forms are modified descendants of the earlier.  And the latter supposition is so vastly more probable than the former, that rational men will adopt it, unless satisfactory evidence to the contrary can be produced.  The objection sometimes put forward, that no one yet professes to have seen one species pass into another, comes oddly from those who believe that mankind are all descended from Adam.  Has any one then yet seen the production of negroes from a white stock, or vice versa?  Moreover, is it absolutely necessary to have watched every step of the progress of a planet, to be justified in concluding that it really does go round the sun?  If so, astronomy is in a bad way.