II. 



Appearance and disappearance of species. — Eeappearance. — Extinction. — Per- 

 sistence of type-structures. — Variation. — Geographical distribution. — Cli- 

 matic zones. — Synchronism of geological formations. 



It is assumed by all, or nearly all, geologists, that every species 

 of animal, broadly speaking, had a definite belonging in the geo- 

 logical scale ; in other words, that its existence was coincident with 

 a certain period in the development of the earth, and with no other. 

 Thus, the well-known and largely-represented brachiopod, Spirifer 

 disjunota (Verneuilii), whose oocuri'ence has been noted in North 

 America, throughout the greater extent of continental and insular 

 Europe, in Asia Minor, China, and New South Wales, is everywhere 

 restricted to the Devonian formation, and is, therefore, distinctive 

 of that period. Similarly, the no less widely disseminated Pro- 

 ductus semireticulatus, a member of the same group of animals, is 

 restricted to the Carboniferous formation. So limited, indeed, ap- 

 pears to have been, in most instances, the range in time of a given 

 species, that the inspection of a single well-determined form will 

 frequently fix, not only approximately but absolutely, the horizon 

 of the deposits whence it was obtained. Belemnitella mucronata, 

 one of the squids, characterises a definite horizon of the Upper 

 Cretaceous ; and among the Ammonites we have numerous instances 

 of specific restriction to special " zones " of even the minor divi- 

 sions of a formation. Limitation of range appears to pertain 

 more strictly to the members of the higher groups of animals than 

 to the lower, or to such forms whose complexity of organisation 

 might be supposed to interfere with a ready accommodation to 

 changing physical conditions of the surroundings. Not one of 

 about one hundred and twenty-five species of fish described from 



