182 GEOLOGICAL DISTEIBUTION. 



the Old Red Sandstone of Great Britain appears to have lived on 

 into the succeeding Carboniferous period, whereas, of the Inverte- 

 brata, comjirising the corals, annelids, echinoderms, crustaceans, 

 and the several classes of the Mollusca, the survivors of the Devo- 

 nian period number no less than twelve per cent. An equally 

 striking case is presented by the Mammalia, where, of the very 

 numerous forms that have been referred to the European and North 

 American Tertiaries, not one is positively known to have passed 

 either from the Eocene division to the Miocene or from the Mio- 

 cene to the Pliocene ; and but a mere handful, if that, from the Plio- 

 cene to the period next succeeding, the Quaternary or Post-Plio- 

 cene. And yet we are aware that, in certain Eocene localities, no 

 less than five per cent, of the molluscan fauna has survived into the 

 present epoch, and as much as thirty to forty per cent, from the 

 Miocene ! Turning to the Invertebrata themselves, we find among 

 the different classes no less striking confirmation of the law of the 

 persistence of the less highly organised specific types over those 

 more highly organised. Taking the sub-kingdom Mollusca, for 

 example, we find, from an examination of the carefully prepared 

 tables of Mr. Etlieridge ^^ on British Paleozoic fossils, that, of the 

 three great classes whose members are not free-swimmers, and who 

 would be consequently most likely to fall under the influence of 

 special physical conditions, the order of persistence is, in most 

 cases, Brachiopoda, Lamellibranchiata, and Gasteropoda — i. c, the 

 lowest first and highest last. Species of brachiopods range from 

 every one of the major formations to the formation next succeed- 

 ing, as from the Cambrian to the Silurian, from the Silurian to 

 the Devonian, from the Devonian to the Carboniferous, and from 

 the Carboniferous to the Permian. The Lamellibranchiata and 

 Gasteropoda, on the other hand, each pass but once from the De- 

 vonian to the Carboniferous. Looking at the numerical develop- 

 ment of the transgressional forms, we find that fourteen per cent, 

 of the Devonian brachiopod fauna reappears in the Carboniferous, 

 thirteen per cent, of the lamellibranch, and eleven per cent, of 

 the gasteropod. Of the equivalent polyzoan fauna the transgres- 

 sional forms constitute thirty-seven per cent. 



Numerous instances of transgressional forms could be cited from 

 the other branches of the animal kingdom, but to no special pur- 

 pose. As a rule, the number of such connecting forms is very 



