FOSSIL FORMS. 



to find the same close correspondence. Genera or families of the 

 above-mentioned classes are found among existing species which 

 are altogether absent from the fossils ; and, on the other hand r 

 numerous genera of fossil animals have no place in the existing 

 animal kingdom. Of about 1000 fossil genera, somewhere about 

 500 are identical with those of existing animals, the other 500 

 being extinct. 



The difference between the present and past creations is, as 

 may be expected, still more remarkable when we come to compare 

 species with species. Thus of 10000 well- ascertained fossil species 

 there are not more than 200 or 300 which still survive. 



56. Though the counterparts of all the principal divisions of the 

 animal kingdom are thus found in the fossil state, they are by no 

 means equally distributed through the strata. Nature, on the 

 contrary, seems to have called them successively into existence, 

 according to their increasing perfection of organisation, the 

 Mammalia, the most perfect of all, being the most recent in date ; 

 and the Zoophytes and Mollusca, the lowest in their organisation, 

 the earliest. 



Thus the first forms of life which appeared during the Silurian 

 period were chiefly confined to the Zoophytes and other classes of 

 the lowest organisation, the only Vertebrates then existing being 

 fishes, and those in very limited numbers. The same forms, for 

 the most part, prevailed upon the globe during the Devonian and 

 Carboniferous periods. During the Permian and Triassic periods 

 animated nature received no other increase than that of a few 

 reptiles. No other classes were added to the creation during the 

 long interval of the Oolitic period ; but the number of species of 

 reptiles, as of all the other classes just mentioned, were con- 

 siderably increased. 



The first appearance of birds was manifested during the Creta- 

 ce'ous period, but they were very limited in number until the 

 Tertiary period. 



It was not till the Tertiary period which immediately preceded 

 the present epoch that Mammalia were created. Birds, reptiles, 

 and fishes augmented in number and variety also during this 

 period, as did various others of the inferior classes, such as Gaste- 

 ropodes and Acephala. 



It must be observed, however, that foot-prints of some Mam- 

 malia have been discovered in the Oolitic strata, and marks, 

 supposed to be those of birds, in the Triassic. 



57. In the following table, compiled from the very extensive 

 tables of Paleontology, which accompany the work of Professor 

 d'Orbigny, we have exhibited the commencement, continuation, 

 and prevalence of the different classes of animals which inhabited 



57 



