238 THE DOCTRINE OF DESCENT. 



elevation of a similar frozen surface in the northern 

 hemisphere, and are surrounded in the Alps by a still 

 existing — in our glacial drift by a scarcely vanished — 

 arctic scene? Or need the conjecture that the almost 

 exclusively graminivorous and insectivorous Marsupials, 

 sloths, armadilloes, ant-eaters, and ostriches, once pos- 

 sessed an actual point of union in a southern continent, 

 of which the present flora of Terra del Fuego, the 

 Cape, and Australia, must be the remains, — need this 

 conjecture raise difficulties at a moment when from their 

 fossil remains Heer restores to our sight the ancient 

 forests of Smith's Sound and Spitzbergen ?" 



HaviuGf ventured to reconstruct the southern conti- 

 nent, with its strange fauna, of which the remains are 

 so widely dispersed, Riitimeyer casts about for more 

 specific evidence in favour of the hypothesis to which 

 the course of the world's formation everywhere gives 

 rise, that fresh-water animals and likewise terrestrial 

 animals came up from the sea. Hence the notably 

 small division of sirenoid fish (Lepidosiren, Proto- 

 pterus), which breathe air during the dry season of the 

 year, must not be considered reptiles adapting them- 

 selves to aquatic life, but the reverse. The organ which 

 in fish served as a hydrostatic apparatus, the swim blad- 

 ders, becomes in them the lung. Thus we must go 

 back from terrestrial to aquatic tortoises, and from them 

 to those denizens of the sea which are allied to the 

 Enaliosaurians, so frequent in the Jurassic strata. The 

 evolutionary and biographical history of the land crabs 

 shows us in the plainest manner how the inhabitant of 

 the sea becomes a terrestrial animal ; a special problem 

 which, as we have already mentioned, Fritz Miiller has 



