Natural selection leaves the subsequent origin and succession of species to the fortuitous concurrence of outward conditions; derivation recognises a purpose in the defined and preordained course, due to innate capacity or power of change, by which homogeneously-created protozoa have risen to the higher forms of plants and animals. The hypothesis of derivation rests upon conclusions from four great series of inductively established facts, together with a probable result of facts of a fifth class; the hypothesis of natural selection totters on the extension of a conjectural condition explanatory of extinction to the origination of species, inapplicable in that extension to the majority of organisms, and not known or observed to apply to the origin of any species. _IV.--Epigenesis or Evolution?_ The derivative origin of species, then, being at present the most admissible one, and the retrospective survey of such species showing convergence, as time recedes, to more simplified or generalised organisations, the result to which the suggested train of thought inevitably leads is very analogous in each instance. If to kosmos or the mundane system have been allotted powers equivalent to the development of the several grades of life, may not the demonstrated series of conversions of force have also included that into the vital form? In the last century, physiologists were divided as to the principle guiding the work of organic development. The "evolutionists" contended that the new being preexisted in a complete state of formation, needing only to be vivified by impregnation in order to commence the series of expansions or disencasings, culminating in the independent individual. The "epigenesists" held that both the germ and its subsequent organs were built up of juxtaposed molecules according to the operation of a developmental force, or "nisus formations." At the present day the question may seem hardly worth the paper on which it is referred to. Nevertheless, "pre-existence of germs" and evolution are logically inseparable from the idea of species by primary miraculously-created individuals. Cuvier, therefore, maintained both as firmly as did Haller. In the debates of 1830 I remained the thrall of that dogma in regard to the origin of single-celled organisms whether in or out of body. Every result of formfaction, I believed, with most physiologists, to be the genetic outcome of a pre-existing "cell." The first was due to miraculous interposition and suspension of ordinary laws; it contained potentially all future possible cells. Cell-development exemplified evolution of pre-existing germs, the progeny of the primary cell. They progagated themselves by self-division, or by "proliferation" of minutes granules or atoms, which, when properly nourished, again multiplied by self-division, and grew to the likeness of the parent cells.