446 M. A. DeCandolle on the Characters which distinguish 



3. TJie cultivated species, especially those which belong to 

 agriculture. 



The number of species relatively to the surface may also be 

 ascertained, particularly that of the species peculiar to the 

 country under examination. 



The ascertainment of these numerical elements is useful, but 

 their employment requires some previous reflections, to which I 

 shall now direct attention. 



5. Variett/ or uniformity of Vegetation. 



The vegetable forms in a countiy may be varied, either by the 

 diversity which they present in different districts, or by the 

 abundance of different forms in each district. In the former 

 case it is advisable to distinguish different regions or zones and 

 to consider them separately. This is done, for example, in 

 mountainous countries, in which several degrees of elevation 

 present plants for the most part different. 



When there is an intimate mixture of vegetable forms in the 

 country under consideration, it is necessary to employ statistical 

 processes. The number of different species is calculated, and 

 afterwards their proportions according to genera and families. 



In order that these numbers should have an equal comparative 

 value in different countries, and even, I may say, an absolute 

 value, it is necessary to acquire an idea of the mode in which the 

 extent of surface modifies the proportions. At the first glance 

 it is seen that the numbers change according to the size of the 

 countiy, and that they change in different proportions, as the 

 species, genera and families occupy average surfaces of very 

 different extent. Both theory and observation agree in showing 

 that it would in fact be incorrect to compare numerical pro- 

 portions founded on regions of too unequal magnitude*. 



If, in the environs of a town, on a space of a hundred square 

 leagues, for instance, we find 1000 species of Phanerogamia be- 

 longing to 400 genera and to 100 natural families, which gives 

 10 species, 4 genera, and 1 family for each square league, and 

 also 2^ species for each genus and 10 for each family, — the pro- 

 portions will be quite different if the circle be extended, even 

 supposing that there is no alteration in the character of the 

 vegetation. We shall arrive much more quickly at the limit of 

 some of the species than at that of the genera, and especially of 

 the families. The species which have disappeared will be replaced 

 by others, more rapidly than we shall meet with new genera or 

 families, in consequence of the relative areas of these groups. 

 Thus, taking a large province in which the supposed town is 



* Many botanists, not much accustomed to the numerical methods, 

 have fallen into this error. 



