PROGRESS OF LIFE. 599 



spect to modern Frogs and Salamanders. The Labyrinthodonts fol- 

 lowed in the expanding line of the Ganoids ; while the Frogs of 

 modern time are an example of the degradation of an old type. Thus 

 it is often the case that tribes have dwindled below the level of their 

 first species. This necessarily follows from the principle stated on 

 page 597. A tribe fitted to the equable climate of Paleozoic time 

 would naturally have become degraded, under a later colder climate or 

 other untoward circumstances. 



8. Peculiarities in the Fauna or Flora of a continent or region con- 

 tinue on through successive geological eras. — Marked examples of a 

 correspondence between the Quaternary and existing life of the con- 

 tinents are mentioned on page 571. Again, the Plants characteristic 

 of the Cretaceous era, in North America, belonged mainly to families 

 that are characteristic of the present time. Cases of this kind are 

 nnmerous ; and exceptions are largely due to migrations on one hand, 

 and extinctions of groups on the other. 



9. The existence of Representative Species in different regions a pos- 

 sible consequence of migration. — On each continent, there have been, 

 in each geological period, not only some living species identical with 

 those of another continent, but also a larger number that were closely 

 similar without being quite identical, and which have hence been 

 called representative species ; at the same time, these species on either 

 continent have continental or regional peculiarities that look like the 

 impress of the region. Such parallel lines of representative species 

 suggest the idea of origin through migration in a former period, and, 

 after that, gradual alteration under the new regional influences. On 

 the Atlantic and Pacific sides of Central America, there are many 

 such representative species ; and they have been regarded as an ex- 

 ample under this principle. 



The continents, as well as the oceans, radiate off from the Arctic 

 zone ; and, consequently, in the period of Glacial cold, Arctic species 

 were forced far south along the several continents and oceans, some 

 even to the Mediterranean (pp. 532, 533); leading thus to the distribu- 

 tion of the same species over widely different meridians and climates, 

 and to the formation, in each, of new varieties. In the Miocene Ter- 

 tiary, there was a comparatively mild climate in the Arctic zone ; and 

 forests abounded. As the climate became cooler with the progress of 

 the Tertiary age (p. 526), the trees of the forests should have spread 

 farther and farther south, along the different continents or meridians* 

 according as the climate in either direction was congenial ; similar 

 kinds along eastern America and eastern Asia, because of the similarity 

 of climate, and other kinds along other lines. A number of the 

 genera and some of the species that then abounded in the Arctic are 



