238 THE DOCTRINE OF DESCENT 



the elevation of a similar frozen surface in the northefn 

 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 conjec- 

 ture raise difficulties at a moment when from their fossil 

 remains Heer restores to our sight the ancient forests of 

 Smith's Sound and Spitzbergen? " 



Having 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 ani- 

 mals came up from the sea. Hence the notably small 

 division of sirenoid fish (Lepidosiren, Protopterus), which 

 breathe air during the dry season of the year, must not 

 be considered feptiles adapting themselves to aquatic 

 life, but the reverse. The organ which in fish served 

 as a hydrostatic apparatus, the swim bladders, becomes 

 in them the lung. Thus we must go back from terres- 

 trial to aquatic tortoises, and from them to those deni- 

 zens 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 be- 

 comes a terrestrial animal; a special problem which, as 

 we have already mentioned, Fritz Miiller has completely 



