492 
GEOGRAPHY. 
BOOK V. 
the equator to the pole follow a similar law of decrement in 
rising from the plains into the mountains of the equator ? Do 
the proportions borne by one family to another vary on the 
same isothermal line ; and are such proportions the same on 
either side of the equator? These are, properly speaking, 
questions of geographical botany : they are connected with 
the most important problems of meteorology, and of the 
physics of the globe in general. 
" In studying the geographical distribution of particular 
forms, we can pause either at a consideration of particular spe- 
cies, genera, or natural families. It often happens that a parti- 
cular species, especially of those kinds which I have called 
social, covers a vast extent of country : such, for instance, are, 
in the north, the heaths and forests of pines; such are, in 
equinoctial America, the assemblages of multitudes of Cactus, 
Croton, Bambusa, and Brathys, of the same species. It is 
curious to examine such instances of multiplication and organic 
developement. We may enquire what species, in a given 
zone, produces the greatest number of individuals ; and we 
may mark the families to which the predominant species 
belong in different climates. 
"In a northern climate, where Compositae and ferns are to 
phaenogamous plants in the relation of one to thirteen, and of 
one to twenty-five (that is to say, when these proportions are 
found by dividing the total number of phaenogamous plants 
by the number of Compositae and ferns), one single species of 
fern may occupy ten times as much land as all the Compositae 
put together. In such a case, ferns would exceed Compositae 
by their mass^ by the number of individuals belonging to par- 
ticular species of Pteris or Polypodium ; but they would not 
exceed them if a comparison were instituted between the dif- 
ferent forms exhibited by the two groups of Compositae and 
ferns, and the sum total of phaenogamous species. As the 
multiplication of all species does not follow a single law, and 
as they do not all produce an equal number of individuals, 
the quotients obtained by dividing the total number of phaeno- 
gamous plants by the number of species of different families 
do not by themselves determine the aspect, or, it might almost 
be said, the nature, of the monotony of vegetation in different 
