CONCLUDING SUMMARY: EXTENT OF BANGE OF VARIATION. 583 



nifera require further elucidation, we feel certain that the six genera represented in the 

 Upper Triassic clay of Chellaston by about 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-water deposits of the Trias period*." 



259. 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, are results 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 

 denizens 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 these present themselves ; so that Messrs. RUPERT JONES and 

 PARKER feel themselves able to pronounce approximatively 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. 



260. In applying the results of the foregoing inquiry to the Animal Kingdom 

 generally, it may be at once conceded that no other group affords anything like the 

 same evidence, on the one hand of the derivation of a 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. But a nearly parallel case, 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 only that their range of variation is extremely wide, but that a 

 large number of reputed genera and species have been erected on no better foundation 



* Quarterly Journal of the Geological Society, November 1860, p. 458. 



