MAMMALIA. 
7. But the land continued to rise^ and the Ilimalayahs, in 
their rise^ also raised that portion of land lying between the 
sources of the Ganges and this great lake. Of course, this cut 
off the exit by the lake into the Ganges. Those individuals of 
Platanista which were out in the waters of the river would 
find themselves cut off from their natural home, and restricted 
to a river-life in the Ganges — a new condition, perhaps, of suf- 
ficient importance to induce a second change into Platanista 
gangetica. 
8. The lake, cut off from its exit by the Ganges, continues to 
rise until it again overflows elsewhere, and this time finds an 
exit where the mouth of the Indus now is, and the Indus flows 
through the midst of it : old channels show that the Indus once 
so flowed, and not, as now, to the west of it. The surviving 
shoals of Platanista, in their turn, would find their lake-life 
turned into a river one, and Platanista indi is the result. 
The author has no doubt that Inia amazonica was produced 
by a similar concurrence of circumstances, with the exception 
that there it was not a double event, but only a single-bar- 
relled^'’ phenomenon, at least so far as species is concerned. 
The author holds that the transformation of old species into 
new is usually (if not always) effected through the medium of 
large numbers of individuals ; chance colonists, being of course 
solitary or few in number, would not undergo this change until 
their numbers had sufficiently multiplied. He would there- 
fore infer, wherever individuals belonging to the same identical 
species occur in different lands (always excepting polar districts 
and those where the physical condition is uniform) , that their 
presence is probably due to colonization ; and where the species 
are representative, that there is a presumption that the land in 
which they occur must at some former period have been con- 
nected with that of the typical species. 
The author treats then of the geographical changes of the 
globe since the secondary epoch, and commences a series of 
chapters on what has been styled by Van der Hoeven Geo- 
graphical Zoology,^^ taking the different families and genera in 
systematic order, and giving an account of their geographical 
range. This forms the greater part of the work, viz. from p. 56 
to p. 296. Two chapters are devoted to the distribution of 
Man, whom he divides into two races only, the white and the 
black. This part contains also remarks on the affinities of 
certain genera, which must be regarded as expressing merely 
the individual views of the author, whose original researches 
into their structure are evidently very limited. 
Zoological Geography (or that part in which we might 
have expected interesting results by generalizing the numerous 
facts so diligently collected by the author in the preceding 
chapters) comes in for a very small share (pp. 304-314). He 
