308 PHYSIOLOGICAL BOTANY. PART If. 



which embrace a greater diversity of stations will, ceteris 

 paribus, also contain a greater number of species. Those 

 which are more strictly isolated from each other are 

 not so likely to interchange their species ; and hence it 

 is observed, that a given space on a continent generally 

 contains a far greater number of species than an equal 

 space in an island. An elevation of temperature is 

 favourable to the greater number of species, as we find by 

 the fact that the number at different latitudes increases 

 as we approach the equator. The genera and families 

 also seem to obey a similar law; but we scarcely possess 

 sufficient information to speak positively as to the pro- 

 portion in which the relative rate of their increase takes 

 place. It does not appear that the same proportion of 

 genera to species is maintained in different latitudes : 

 for instance, the species in Sweden are to those in 

 France as one to three; whilst the genera are as one to 

 two. 



(3 14-.) Proportion of Species in each Class, in dif- 

 ferent Regions. If a botanist collect indiscriminately 

 all the plants he meets with, in any region he may be 

 examining, he will most probably be soon able to obtain 

 a very close approximation to the relative proportion 

 which the species of each of the three classes, and 

 many of the orders bear to each other, long before he 

 has obtained an accurate notion of the whole number of 

 species which the region possesses. So far as calcu- 

 lations have hiterto been made, the following general 

 laws appear to be correct ; and it is not likely that they 

 will be modified by any additional information which 

 future researches may procure. 



1. The proportion of cryptogamic to phanerogamic 

 species increases as we recede from the equator. 



2. The proportion of Dicotyledones to Monocotyle- 

 dones increases as we approach the equator. 



3. The absolute number of species, and also the 

 proportion of woody species to the herbaceous, increases 

 as we approach the equator. 



4. The number of species either annual or biennial 



