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 ?” 
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 
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 
