20 THE WORLD OF LIFE CHAP. 



tributed than those whose seeds are not winged. These 

 facts certainly prove that the dispersal of seeds by wind or 

 by birds has been brought about for the purpose of securing 

 ample means of reproduction within the area to which the 

 whole plant has become specially adapted, not to facilitate 

 its transmission to distant lands or islands which, only in 

 a very few cases, would be suited for its growth and full 

 development. Very extensive dispersal must, therefore, in 

 most cases be looked upon as an adventitious result of general 

 adaptation to the conditions in which a species exists. 



De Candolle's work also treats very fully the subject of 

 the comparative preponderance of the various natural orders 

 of plants in different regions or countries. This mode of 

 studying plant-distribution was introduced by our greatest 

 English botanist, Robert Brown, and it is that most generally 

 used by modern botanical writers on distribution. It con- 

 sists in the characterisation of the vegetation of each region 

 or district by the proportionate abundance in species belong- 

 ing to the different natural orders. 



This is used in many different ways. In one the 

 minimum number of orders whose species added together 

 form one-half of the whole flora are given. Thus, it was 

 found that in the Province of Bahia (Brazil) the 1 1 

 largest natural orders comprise half the whole number of 

 species. In British Guiana 12 orders are required, and in 

 in British India 17. Coming to temperate regions, in 

 Japan there are 16, in Europe 10, in Sweden 9, in Iceland 

 and in Central Spain 8. The general result seems to be 

 that those regions which are very rich in their total number 

 of plants require a larger number of their preponderant 

 orders to make up half the total flora ; which implies that 

 they have a larger proportion of orders which are approxi- 

 mately equal in number of species. 



Another mode of comparison is to give the names of the 

 first three or four, or even ten or twelve, of the orders which 

 have the greatest number of species. It is found, for 

 example, that in equatorial regions Leguminosa:: usually 

 come first, though sometimes Orchids are most abundant ; 

 in temperate regions the Composites or the Grasses ; and in 

 the Arctic, Grasses, followed by Cruciferae and Saxifrages. 



