The criticism is all embodied in two propositions--namely, (_a_) that the theory of natural selection carries with it, as a "necessary consequence," the doctrine that survival of the fittest has been the cause of the origin of _all_ species; and (_b_) that therefore it amounts to one and the same thing whether we define the theory as a theory of species or as a theory of adaptations. Now, as a mere matter of logical statement, it appears to me that both these propositions are unsound. As regards the first, if we hold with Darwin that other causes have co-operated with natural selection in the origination of some (i. e. many) species, it is clearly no part of the theory of natural selection to assume that none of these causes can ever have acted independently. In point of fact, as we have seen in the foregoing chapters, such has probably and frequently been the case under the influences of isolation, climate, food, sexual selection, and laws of growth; but I may here adduce some further remarks with regard to yet another possible cause. If the Lamarckian principles are valid at all, no reason can be shown why in some cases they may not have been competent _of themselves_ to induce morphological changes of type by successive increments, until a transmutation of species is effected by their action alone--as, indeed, Weismann believes to have been the case with all the species of Protozoa[148]. That such actually has often been the case also with numberless species of Metozoa, is the belief of the neo-Lamarckians; and whether they are right or wrong in holding this belief, it is equally certain that, _as a matter of logical reasoning_, they are not compelled by it to profess any _disbelief_ in the agency of natural selection. They may be mistaken as to the facts, as Darwin in a lesser degree may have been similarly mistaken; but just as Darwin has nowhere committed himself to the statement that _all_ species must _necessarily_ have been originated by natural selection, so these neo-Lamarckians are perfectly logical in holding that _some_ species may have been wholly caused by the inheritance of acquired characters, as _other_ species may have been wholly caused by the natural selection of congenital characters. In short, unless we begin by assuming (with Wallace and against Darwin) that there _can be no other cause_ of the origin of species than that which is furnished by natural selection, we have no basis for Professor Huxley's statement "that every species has been originated by selection"; while, if we do set out with this assumption, we end in a mere tautology. What ought to be done is to prove the validity of this assumption; but, as Professor Huxley makes no attempt to do this, his criticism amounts to mere begging of the question. [148] Since the above was written Professor Weismann has transferred this doctrine from the Protozoa to their ancestors.