ANNOTATIONS AND ADDITIONS. 109 



The forms of organic beings are in reciprocal dependence 

 on each other. In the unity of nature these forms limit 

 each other according 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 Giumacese, Leguminosse, or 

 Compositse, we may with a tolerable degree of probability 

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

 phanerogamse of the country, and also as to the number 

 of species belonging to the rest of the leading families of 

 plants. The number of Cyperoidse determines that of 

 Composite, and the number of Composite that of Legumi- 

 nosae; 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 different systems of vegetation, what harvest 

 may still remain to be reaped in the several families. 



The comparison of the numerical ratios of families in 

 different already well explored zones, has conducted me to 

 the recognition of laws according to which, in proceeding 

 from the equator to the poles, the vegetable forms consti- 

 tuting a natural family decrease or increase as compared 

 with the whole mass of phanerogams belonging to each 

 zone, j We have here to regard not only the direction of 

 the change (whether an increase or a decrease), but also its 

 rapidity or measure. We see the denominator of the 

 fraction which expresses the ratio increase or decrease : let 

 us take as our example the beautiful family of Leguminosae, 

 which decreases in going from the equinoctial zone towards 

 the North Pole. If we find its proportion or ratio for the 



