BOOK V. 
GEOGRAPHY. 
493 
quarters of the world. A traveller is often surprised at the 
continual repetition of individuals of one species, and of the 
masses of such individuals which are continually occurring ; 
but he has equal reason to wonder at the rarity of other spe- 
cies which are useful to mankind. Thus, in countries where 
whole forests are formed by Rubiaceae (Cinchonacese), Legu- 
minosae, and Terebinthaceae, the Cinchonas, logwood, and 
basalm trees are comparatively very rare. 
" In the consideration of species, the subject may also be 
viewed in an absolute manner with reference to the number of 
species which prevail in particular zones. This interesting kind 
of comparison has been made in M. De Candolle's grand work, 
and Mr. Kunth has carried it into effect with more than 3500 
Compositae now known. It does not, indeed, indicate what 
families predominate, in a given degree, over other phaenoga- 
mous plants, either with regard to the number of species, or 
the mass of individuals ; but it determines the numerical 
relations of species of the same family in different latitudes. 
The most varied forms of ferns, for instance, are found in the 
tropics ; it is in the mountainous, temperate, humid, and 
shady regions of those parts of the world, that the family of 
ferns produces the greatest number of species. In the tem- 
perate zone there are fewer than in the tropics, and the total 
number continues to decrease as we approach the pole ; but 
as a cold country, Lapland, for instance, produces species 
that have a greater power of resisting low temperature than 
the great mass of phaenogamous plants, it happens that, in 
Lapland, the relative proportion borne by ferns to the rest of 
the flora is greater than in France or Germany. The nume- 
rical relations which appear in the tables that are now about 
to be produced, are entirely unlike the relations indicated 
hy an absolute comparison of the species that vegetate under 
different parallels of latitude. The variation which is observ- 
able in proceeding from the equator to the poles is conse- 
quently different in those two methods. In that of fractions, 
which is adopted by Mr. Brown and myself, there are two 
causes of variation ; that is to say, the total numbers of phae- 
nogamous plants do not vary in passing from one parallel of 
latitude, or rather from one isothermal zone to another, in 
