412 NOTICES OF BOOKS. 
exceptions to its general aspect, and all the species which it contains 
are worthy of a careful analysis. We find, on the data contained in the 
list itself and in such German floras as are at hand, that of the 125 
species enumerated 52 are natives of Saxony, and 29 only of Ham- 
burgh, Holstein, or Denmark. Both these numbers are possibly rather 
too low; but if we make an allowance for naturalized species, a con- 
siderable deduction will be required, so that these numbers suffice 
to show the general southern nature of the forms. Only 50, or less 
than one-half, are natives of Britain; and of these at least 28 are con- 
fined to the extreme south of our island, and are therefore affected in 
their distribution by some cause or other, in the same way as in Ger- 
many. Several of the others, such as Wahlenbergia hederacea and Hy- 
pericum Elodes are western forms; and scarce one, except Carea stricta 
and Potamogeton densus, is a common British plant. 
While therefore we regard such careful observations as those of our 
author as of great value to the student of geographical botany, we feel 
by no means disposed to depreciate so much as he does the influence 
of climatic conditions on the vegetation on our globe. It is no doubt 
difficult to explain in what manner the organization of a plant is af- 
fected by nice shades of difference of temperature or humidity. The 
fact is, however, well established as between tropical and temperate cli- 
mates; and the usual explanation seems quite satisfactory, that impaired 
vitality at last becomes incapable of contending against more powerful 
competitors in the struggle for space which is constantly going on be- 
tween one species and another. x 
Dr. Hoffmann draws, as a further conclusion from his comparative 
. list of species, that the highest levels at which plants are found on the 
hills surrounding the Rhine valley will in a great many instances be 
found to correspond at great distances; and he is of opinion that this 
result is due to the level of the lake or sea which at the time of their 
‘introduction occupied the valley, believing, as we have seen, that trans- 
‘mission by means of water is the only energetic means of dispersion of 
plants, and that they are incapable of extending to any distance in any 
other way. Impressed with this view, he takes a rapid survey of the geo- 
graphical history of Germany since the beginning of the tertiary epoch, 
and speculates on the relation of the flora to the periods of emergence. 
Into this it is unnecessary to follow him, as such speculations are not 
capable of any close reasoning in the present state of our geological 
