SUCCESSION OF SIMILAR TYPES. 171 



terised by its Sloths, Armadillos, and Anteaters. In addition to 

 these gigantic forms, small and extinct species have been found in 

 the bone caves of Brazil, and some of these are so nearly related to 

 the living forms that we may assume them to have been their 

 ancestors. 



This law of the " succession of similar types " in the same localities 

 is also exemplified by the Mammalia of New Holland ; for in the 

 bone caves of that country are found many species nearly allied to 

 its present Carnivora. The same law holds good for the gigantic birds 

 of New Zealand, and, as Owen and others have shown, for the Mam- 

 malia of the Old World, which, indeed, is continuous by the circum- 

 polar region with North America ; and ancient types were able, in the 

 tertiary period, to pass into North America, and vice versa by that 

 way. The presence of Central American types (Didelphys) in the 

 early and middle tertiary formations of Europe is to be explained 

 in the same way. It is even more difficult to distinguish the regions 

 of distribution of the animals of that time than of those of the later 

 tertiary period. 



The evolution of the ancient forms into those of the present 

 time was effected in the case of lower, simply organised animals 

 at a much earlier period than in the case of higher organisms. 

 Rhizopods, indistinguishable from species living at the present 

 time (globigerina ooze) were already living in the Cretaceous period. 

 The deep sea explorations * have accordingly yielded the interesting 

 result, that certain Sponges, Corals, Molluscs, and Echinoderms now 

 living in the deep sea existed in the Cretaceous period. We meet 

 with a number of living species of Molluscs in the oldest tertiary 

 period, though the mammalian fauna of this period differs completely 

 from that of the present day. The greater number of species of 

 Molluscs found in the recent tertiary period resemble those of the 

 present day, but the Insects of that formation differ considerably from 

 living species. 



On the other hand, the Mammalia, even in the post-pliocene 

 (diluvial) deposits, differ in part both in genera and species from 

 those of the present day, although a number of forms have been 

 preserved through the glacial period. On tin's account, and on 

 account of the relative completeness of the tertiary remains, it is 



* (Rliizocriiws Lofoten*:!* Ajtiiirri/tifrx, Plewrotomaria, Siplionia, Micr<i*f< /. 

 Poinofiirix. etc.) Types of earlier and even of the older geological formations 

 have been found preserved in the depths of the oceau, which, in spite of the great 

 pressure, the want of light and deficiency in gaseous contents of the water, are 

 more suited to the development of animal life than was formerly believed. 



