That this is, to a certain extent, a true explanation of the existence at the present day of _low_ forms of animals is proved by the fact that we find in very ancient strata fossil remains of animals which differ, ever so little, from particular animals existing at the present day; for instance, the Brachiopods (lamp-shells), Lingula and Terebratula, the King-crabs, and the Pearly Nautilus are found living at the present day, and are also found with no appreciable difference in very ancient strata of the earth’s crust; strata deposited so long ago that most of the forms of life at present inhabiting the earth’s surface had not then been brought into existence, whilst other most strange and varied forms occupied their place, and have now for long ages been extinct. Whilst we are thus justified by the direct testimony of fossil remains in accounting for _some_ living forms on the hypothesis that their peculiar conditions of life have been such as to maintain them for an immense period of time _in statu quo_ unchanged, _we have no reason for applying this hypothesis, and this only_, to the explanation of all the more imperfectly organised forms of animal or plant-life. It is clearly enough possible for a set of forces such as we sum up under the head “natural selection” to so act on the structure of an organism as to produce one of three results, namely these; to keep it _in statu quo_; to increase the complexity of its structure; or lastly, to diminish the complexity of its structure. We have as possibilities either BALANCE, or ELABORATION, or DEGENERATION. Owing, as it seems, to the predisposing influence of the systems of classification in ascending series proceeding steadily upwards from the “lower” or simplest forms to the “higher” or more complex forms,—systems which were prevalent before the doctrine of transformism had taken firm root in the minds of naturalists, there has been up to the present day an endeavour to explain every existing form of life on the hypothesis that it has been maintained for long ages in a state of Balance; or else on the hypothesis that it has been Elaborated, and is an advance, an improvement, upon its ancestors. Only one naturalist—Dr. Dohrn, of Naples—has put forward the hypothesis of Degeneration as capable of wide application to the explanation of existing forms of life;[4] and his arguments in favour of a general application of this hypothesis have not, I think, met with the consideration which they merit.