CONCLUDING- SUMMAEY: — ^EXTENT OE EANGE OF VAEIATION. 
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 feat ares 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 
shaUower-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 atfect the prevalence of particular types, and to modify 
the forms under which these present themselves; so that Messrs. Fupeet Joxes and 
Paekee 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, Glohigerina. 
260. In applying the results of the foregoing inquiry to the Animal Kingdom 
generally, it may be at once conceded that no other group afibrds 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. 
