100 PHYSIOGNOMY OF T'LANTS. 



in the present state of the organic life of our globe, we may 

 yet attempt an approximate method by which we may find 

 some probable et lowest limits" or numerical minima. Since 

 1815, I have sought, in arithmetical considerations relating 

 to the geography of plants, to examine first the ratios which 

 the number of species in the different natural families bear 

 to the entire mass of the phsenogamous vegetation in coun- 

 tries where the latter is sufficiently well known. Robert 

 Brown, the greatest botanist among our cbtemporaries, had 

 previously determined the numerical proportions of the 

 leading divisions of the vegetable kingdom ; of Acotyledons 

 (Agamse, Cryptogamic or cellular plants) to Cotyledons 

 (Phanerogamic or vascular plants), and of Monocotyledonous 

 (Endogenous) to Dicotyledonous (Exogenous) plants. He 

 finds the ratio of Monocotyledons to Dicotyledons in the tro- 

 pical zone as 1 : 5, and in the cold zones of the parallels of 

 60N.and55S.latitude,as] : 2J. (Robert Brown, General 

 Remarks on the Botany of Terra Australis, in Flinders' 

 Voyage, vol. ii. p. 338.) The absolute number of species in 

 the three leading divisions of the vegetable kingdom are 

 compared together in that work according to the method 

 there laid down. I was the first to pass from these leading 

 divisions to the divisions of the several families, and to con- 

 sider the ratio which the number of species of each family 

 bears to the entire mass of phsenogamous plants belonging 

 to a zone of the earth's surface. (Compare my memoir 

 entitled " De distribution geographica Plantarum secundum 

 coeli temperiem et altitudinem montium, 1817, p. 24-44 ; 

 and the farther development of the subject of these numerical 



