PREFACE. ix 



tion which attends the doctrine of distinct specific creations when applied to a group in which 

 scarcely any two individuals are alike. The case, in fact, is very analogous to that of the 

 relationship between the various members of the family of Mankind ; for whilst the historical 

 evidence of actual change in them is so incomplete, as well as so limited in its range, as to be 

 quite inadequate of itself to establish their community of descent, yet when that evidence is 

 considered in its relations to analogous facts drawn from the far greater variations of domesti- 

 cated animals, and to the manifold gradations by which the extreme types are connected, 

 physiologists of the highest eminence have felt themselves justified in accepting that commu- 

 nity as probable. Now the modifications which any single type of Foraminifera must have 

 undergone, to give origin to the whole series of diversified forms presented by that group, are 

 not greater in comparison with those of which we have direct evidence, than arc those which 

 the advocate for the specific unity of the Human Races has no hesitation in assuming as 

 the probable account of their present divergence. 



This view of the case derives great force from the fact, which constitutes the special 

 feature of interest which this group has for the Geologist, that there is strong reason to 

 regard a large proportion of the existing Foraminifera as the Jirecf lineal descendants of those 

 of very ancient geological periods. This doctrine was first advanced by Prof. Ehrenberg 

 in regard to a considerable number of Cretaceous forms ; and has since been fully confirmed 

 and extended as regards the Tertiary fauna by the admirable researches of Messrs. Rupert 

 Jones and Parker on the Rhizopodal Fauna of the Mediterranean, as well as by my own com- 

 parison of the recent and fossil types of Orbitolites, OrbicuUiia, Alveolina, Ojierculbia, and 

 Calcarina ; and it has been shown to be applicable also to the Secondary fauna, as far back 

 as the upper part of the Triassic system, by the remarkable results of the investigations of 

 ni)'' coadjutors in regard to a well-preserved sample of it. 



It can scarcely 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 diversity of external conditions ; 

 so that, on the one hand, they pass unharmed through changes in those conditions which are 

 fatal to beings of higlier structure and more specialized constitution ; wliilst, 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 merel}^ 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, wliicli 

 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 



b 



