"In comparing the several stages in the very interesting development of the _cyanaea aurita_ to the infusoria and polypes, it must be understood that such comparisons are warranted only by a similarity of outward form, and of the instruments of locomotion and prehension. The essential internal organization of the persistent lower forms of the _zooephyta_ is entirely wanting in the transitory states of the higher ones. A progress through the inferior groups is sketched out, but no actual transmutation of species is effected. The young medusa, before it attains its destined condition of maturity, successively resembles, but never becomes, a polygastrian, a rotifer, and a bryozoon."--p. 112. "Thus every animal in the course of its development typifies or represents some of the permanent forms of animals inferior to itself; but it does not represent all the inferior forms, nor acquire the organization of any of the forms which it transitorily represents. Had the animal kingdom formed, as was once supposed, a single and continuous chain of being, progressively ascending from the monad to the man, unity of organization might then have been demonstrated to the extent in which the theory has been maintained by the disciples of the Geoffroyan school."--p. 370. If these similarities of structure in the germ had any bearing on the subject, they would indicate the possibility only of retrogression in the scale. Of course, the immature ovum, arrested in its development, could not form a more perfect being than its parent. There is no pretence that the embryo, at any stage of its progress, images an animal of a higher grade than its own family. Then what aid do these similarities of structure afford to the theory, that all the higher organisms have been evolved by successive steps out of the lowest monad? At the best, you have only shown, that a _retreat_ is possible; you have still to point out any likelihood, even the remotest, of an _advance_ in the scale of being. There is no fact whatever to confirm the supposition, that birth may possibly be delayed till the animal be developed into one of a higher species; and the law of immature births seems to be, that, if the offspring escapes at all--for there is great risk consequent on such an accident,--it becomes as perfect as its progenitors. Nature seems to guard the distinctions between the several races with peculiar care; so far as we know, monsters either do not survive their birth, or are incapable of continuing their kind, or in the course of a single generation are reunited to the original family.