106 PHYSIOGNOMY OF PLANTS. 



this kind,, (i. e. the entire quantity of phsenogamous 

 plants in each of the different Floras divided by the number 

 of species in each family) in my Prolegomenis de distri- 

 butione geographica Plantarum, in 1817 ; and in the 

 Memoir on the distribution of plants over the Earth's 

 surface, subsequently published in the French language, I 

 corrected my previously published numbers by Robert 

 Brown's great works. In advancing from the Equator 

 to the Poles, the ratios taken in this manner vary considera- 

 bly from the numbers which would be obtained from a 

 comparison of the absolute number of species belonging to 

 each family. We often find the value of the fraction in- 

 crease by the decrease of the denominator, while yet the ab- 

 solute number of species has diminished. In the method 

 by fractions, which I have followed as more instructive in 

 reference to the geography of plants, there are two varia- 

 bles; for in proceeding from one isothermal line, or one 

 zone of equal temperature, to another, we do not see the 

 sum total of all the phanerogam se change in the same pro- 

 portion as does the number of species belonging to a parti- 

 cular family. 



We may, if we please, pass from the consideration of 

 species to that of divisions formed in the natural system of 

 botany according to an ideal series of abstractions, and 

 direct our attention to Genera, to Families, and even to the 

 still higher, i. e. more comprehensive, Classes. There are 

 some genera, and even some entire families, which belong 

 exclusively to particular zones of the Earth's surface ; and 

 this not only because they can only flourish under a particular 



