Page:Natural History Review (1861).djvu/199

Rh And it appears to me a justifiable inference from this fact, that the wide range of forms which this group contains, is more likely to have come into existence as a result of modifications successively occurring in the course of descent from a small number of original types, than by the vast numbers of originally distinct creations which on the ordinary hypothesis would be required to account for it.

The greater part of my first memoir was devoted to the investigation of a single type, Orbitolites; and I there showed, that not only as regards the size, shape, and other external characters of the organism as a whole, but even as regards the size and form of its elementary parts, in which greater constancy might be expected, is there so great a variation (the most marked diversities being apparent even in different parts of the same specimen), that all attempts to found specific distinctions upon such variations are utterly futile. But further, I showed that a distinction on which almost any naturalist would feel justified in relying, as of specific if not of generic value, that between the simple type in which all the cells are arranged on only one plane, and the complex type in which there are two superficial planes more or less strongly differentiated from the median, is no less invalid. For although these types are usually distinguishable the one from the other without the least difficulty, yet they are often combined in the same individuals, and this in such a variety of modes, that the transition from the simple to the complex may be clearly seen, by the comparison of a sufficient number of specimens, to be by no means attributable to a mere advance of age. Further, having been furnished (by the kindness of Mr. H. J. Carter) with specimens of the Scindian fossil which presents the characters ascribed by M. D'Orbigny to his genus Cyclolina, I find, as I had anticipated, that this genus is founded upon a mere variety of Orbitolites, in which the character of the surface-marking is more than ordinarily cyclical. Not merely, however, does the range of variation of this type confound the ordinary distinctions of systematists in regard to species and genera; it extends also to that difference in plan of growth which has been assumed by M. D'Orbigny of such fundamental importance, as justly to constitute the essential difference between his two orders Cyclostègues and Helicostègues. For, as I have shown, although Orbitolites is typically cyclical from its commencement, yet specimens frequently present themselves in which its early development has taken place so completely on