ANNOTATIONS AND ADDITIONS. 299 



whether the writer is speaking of the absolute number of species, 

 and its increase or decrease with the change of latitude ; or whether 

 he means that the family in question prevails over other families of 

 plants as compared with the entire number of phanerogamae of which 

 a Flora consists. The impression of prevalence as conveyed by the 

 eye depends on relative quantity. 



Terrestrial physics have their numerical elements, as has the Sys- 

 tem of the Universe, or Celestial Physics, and by the united labors 

 of botanical travellers we may expect to arrive gradually at a true 

 knowledge of the laws which determine the geographical and clima- 

 tic distribution of vegetable forms. I have already remarked that 

 in the temperate zone the Compositae (Synantherese), and the Glu- 

 maceae (including under this latter name the three families of 

 Grasses, Cyperoidae and Juncacese), make up the fourth part of all 

 phaenogamous plants. The following numerical ratios are the results 

 of my investigations for 7 great families of the vegetable kingdom 

 in the same temperate zone. 



Glumaceae -\ (Grasses alone T 'y) 



Composite 



Leguminosse yg 



Labiatse o 1 ? 



Umbelliferae ^ ff 



Amentaceae (Cupuliferse, Betulineae, and Salicineae) fa 



Cruciferae T V 



The forms of organic beings are in reciprocal dependence on each 

 other. In the unity of nature these forms limit each other accord- 

 ing to laws which are probably attached to periods of long duration. 

 If on any particular part of the globe we know with accuracy the 

 number of species of one of the great families of Grlumacae, Legumi- 

 nosaa, or Compositae, we may with a tolerable degree of probability 

 form approximative inferences, both as to the sum of all the phane- 

 rogamae of the country, and also as to the number of species belong- 

 ing to the rest of the leading families of plants. The number of 

 Cyperoidae determines that of Compositae, and the number of Com- 

 positae that of Leguminosas ; they even enable us to judge in what 

 classes or orders the Floras of countries are still incomplete, and 

 teach us, if we are on our guard against confounding together very 



