410 NOTICES OF BOOKS. 
starting from the foundation that the present vegetable world is not to - 
be considered as having been created on the spot on which it now 
grows, but that, as in the animal kingdom, species must be considered 
to have descended from more or less remote tertiary periods, since 
which the surface of our continent has been subject to a very great 
amount of change. The whole of Germany, he states, is considered to 
have arisen gradually from the sea since the beginning of the tertiary 
epoch, and the great valleys are believed to have been occupied during 
the period of emergence by arms of the sea or fresh-water lakes, gradu- 
` ally diminishing in size and in the height of water-level. In investigating 
the mode of entrance of plants into the tracts laid bare by the elevation 
of the land, he feels compelled to select one of two principal modes of 
transmission—either by land or by water; and both by à priori reasoning 
and by a series of illustrative facts, be believes that he has proved that 
it is by means of water-carriage that the principal immigration of plants 
has taken place. The à priori arguments are naturally of less weight 
and novelty, relating chiefly to the non-correspondence between facilities 
of aerial transport and rapidity of introduction, and to the small amount 
of dispersing power which is exhibited by plants in isolated localities. 
These are to us not convineing; but we need not here controvert them 
in detail, as counter-facts in abundance will suggest themselves to 
all students of geographical botany. The illustrative facts which the 
author has brought to bear upon the question of water-transport are, 
on the contrary, the most interesting and valuable part of the work be- 
fore us. They consist of a detailed list of 125 species of plants, which 
are common in all parts of the district of the Middle Rhine and of its 
tributaries, up to an elevation of 1000 feet above the level of the sea, 
but which are not found in any part of the middle or upper valleys of 
the Weser, a river whose sources have no connection with the moun- 
tains of South Germany: these he therefore supposes to have immi- 
grated by water from the high lands of Switzerland and South Ger- 
many, after the ridge of land which separates the Weser valley from 
that of the Rhine had been elevated above the level of the sea. 
On the preparation of these lists of species great pains have evidently 
been bestowed, and they may (as far as the German flora is concerned) 
be considered as critically accurate. To enable the author to ascertain 
_ the main facts of geographical distribution, the West German localities 
of every species of phanerogamous plant which is indigenous in the 
