For the vast majority of cases in which the commencement of life and organization falls under our notice being confessedly those, not of primary production, but of mediate reproduction, it is reasonable to believe that the same law governs all cases alike, whether we have been able or not to trace the origin of life to the principle of propagation, the few apparent exceptions being sufficiently accounted for by our imperfect knowledge of the causes and conditions on which they depend.
Besides, the argument from analogy in favor of a primary production of life by natural causes, in so far as it is founded on the present law of hereditary transmission, is radically defective, since the two cases are widely different; the one presupposing _a primary organism of the same kind_, from which others are evolved by a law of natural succession, the other exhibiting life as a new product, resulting not from any prior organism, but from the action of _causes of a totally different kind_, which are not known to be capable of giving birth either to vegetable or animal organisms under the actual constitution of Nature.
But suppose, even, that the _Acarus Crossii_ were admitted to be a real product of Galvanic action on the silicate of potash, and an undeniable instance of "a non-generative origin of life," how would the illustrative example accord with the author's general theory?
It might afford a specimen of aboriginal production; but how would it fit in with his favorite doctrine of _a gradual and progressive advancement_ from the lower to the higher forms of organization?
The _Acarus_, at first supposed to be a new and hitherto unknown creature, is now acknowledged to be one of a very familiar species,--a species which may have deposited its ova, and propagated its kind, since the commencement of the present order of things, and whose eggs might very well resist the action even of nitrate of copper, since the creature itself could live in that poisonous mixture.

Moreover, it belongs, in point of organization, to one of the highest orders of organisms; not to the _radiata_, not to the _mollusca_, but to the highest type of the _articulata_, the nearest to the _vertebrata_.

Had it been a monad,--a mere living cell,--which Galvanism evolved from the solution, and had this primary product developed itself afterwards in various forms, according to the ascending scale of a progressively improving organization, it might have accorded admirably with the twofold doctrine of spontaneous generation and transmutation of species; but, unfortunately, the first process is so perfect, in the present instance, as to leave little room for the second, and we are almost tempted to hope that perhaps the clumsy and troublesome expedient of a transmutation of species may yet be superseded by the discovery of some method,--we know not what,--whereby not only the _articulata_, but the _vertebrata_, and even Man himself, may be immediately produced by some new combination of Nature's elemental laws![47] We have given prominence, in the first instance, to the doctrine of "spontaneous" or "aboriginal" production, because it constitutes an indispensable part of the Theory of Development, and because we believe that, were this clearly understood, that theory would soon sink into general discredit or total oblivion, like the kindred speculations of Anaximander and Anaxagoras, of the old Ionic School.

The experiments of Ehrenberg, instituted with the view of testing the doctrine of spontaneous generation, may be said to have decided the whole question.
They did not succeed, indeed, in explaining every apparently exceptional case, for some of the facts are still obscure, and will probably continue to be so, notwithstanding every extension of microscopic power, just as, in the analogous case of the Nebulae, the increase of telescopic power has enabled us to resolve not a few of them into clusters of stars, while it has served to bring others yet unresolved within the range of our vision.

But they were sufficient, at least, to show that, as far as our clear knowledge extends, the one uniform law, "_Omne vivum ex ovo_," universally prevails, and that the whole analogy of Nature, in so far as its constitution has been ascertained, is adverse to the doctrine of spontaneous generation.