Page:Natural History Review (1861).djvu/210

198 thirty varieties, stand really in the place of ancestral representatives of certain existing Foraminifera, that they put on their several subspecific features in accordance with the conditions of their place of growth, just as their posterity now do; and that although we have in this instance met with only the minute forms of a 700-fathoms mud-bottom, yet elsewhere the contemporaneous fuller development of these specific types may be found by careful search in other and shallower deposits of the Trias period".

It can scarcely, I think, be questioned that such a continuity of the leading types of Foraminifera maintained through so long a series of geological periods, and the recurrence of similar varietal departures from those types, is a result of the facility with which creatures of such low and indefinite organization adapt themselves to a great variety of external conditions; so that, on the one hand, they pass unharmed through changes in those conditions which are fatal to beings of higher structure and more specialized constitution; whilst on the other, they undergo such modifications, under the influence of those changes, as may produce a very wide departure from the original type.

Thus we have found strong reason for regarding temperature as exerting a most important influence in favouring, not merely increase of size, but specialization of development; all the most complicated and specialized forms at present known being natives either of tropical or of sub-tropical seas, and many of these being represented in the seas of colder regions by comparatively insignificant examples, which there seems adequate reason for regarding as of the same specific types with the tropical forms, even though deficient in some of their apparently most important features. The depth of the sea-bottom seems also to affect the prevalence of particular types, and to modify the forms under which they present themselves; so that Messrs. Jones and Parker feel themselves able to pronounce approximately as to the depth of water at which a deposit of fossil Foraminifera may have been formed, by a comparison of its specific and varietal types with those characterizing various depths at the present time. And it is specially worthy of note, that in the greatest depths of the ocean from which Foraminifera have been brought by deep-sea soundings, these belong almost exclusively to one type, Globigerina.

Now it may be at once conceded that no other group in the Animal kingdom affords any thing like the same evidence, on the one hand, of the derivation of a vast multitude of distinguishable forms from a few primitive types, and on the other, of the continuity of those types through a vast succession of geological epochs. A somewhat parallel case, however, as regards the first of these points, is presented by certain of the humbler groups of the Vegetable kingdom, in which it is becoming more and more apparent, from the careful study of their life-history, not