! [300]. 80 
instances of penttiplicetien and organic development. We may inquire 
-what species, ina given zone, produces the greatest es of individuals ; 
and we may mar rc the families to which the predominant species belong in 
different climates. 
“ In a northern climate, where Composite and ferns are to pheenogamous 
plants in the relation of one to thirteen, and of one to twenty-five, ‘(that i is 
to say, when these. proportions are found by oe the total number of 
papers plants by the number of Composite and ferns,) one single 
ies of fern nh ney occupy ten times as — land as all the Com mpositee 
el together. In sucha case, ferns would exceed Com posite by their mass, 
by the number of individuals belonging to “partenlés species of Pteris or 
Polypodium ; but they would not exceed them ifa comparison were insti- 
tuted between the different forms exhibited by the two groups of Composit » 
and ferns, and the sum total of phanogamons species. As the acolo 
tion of all species does not follow a single law, and as they. do all pro- 
duce an equal number of individuals, the quotients obtained ey dividing 
the total number of Bhefeganihs plants by the number o 
different families do not by themselves determine the aspect, 0 might 
1 said, the nature, of oh sp ony of vegetation He differen, 
quarters of the world. A traveller is often surprised at the continual repe- 
tition of individu als of one Species, a and of the masses of such individuals 
which are continually occurring ; but he has equal reason to wonder at the 
rarity of other species which are useful to mankind. Thus, in countries 
where whole forests are formed by Rubiacez, (Cinchonacew,) Leguminosar, 
and ‘T'erebint panes, the Cinchonas, logw ood, and basalm trees are compar- 
s 
atively ve 
“In the ses a a of species, the mee may also be viewed in an 
absolute manner with reference to the n mber 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 3,500 Composite now known. It does not, indeed, indicate what . 
families predominate, i in a given degree, over other phaenogamous 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. ‘lhe most varied forms of ferns, for instance, are found 
in the tropics; itis in the mountainons, temperate, humid, and sha ady re- 
gions of those parts Of the world, that the family of ferns produces the 
greatest number of species. In the temperate zone there are fewer than in... 
the tropics, and the total number continues to decrease as we approach the é 
. 
= but as a cold country, Lapland, for instance, produces species that . — 
phen ogam nus 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 numerical relations, which appear in the tables that are ne ee 
to be produced, are entirely unlike the relations indicated by an a ; 
comparison 0 of the species that y, vegetate under different parallels of ate. 
The variation which is observable in n proceeding from the equator to 
es, is meapeariontly different in those two methods. In that of fractions, — 
which is adopted by Mr. Brown and myself, there are two causes of varin- 
om: that is to say, sie total numbers of pha:nogamous plants do not vary 
a, passing from one — of latitude, or rather from one is-thermal zone — 
