DISTRIBUTION OF SPECIES 21 



ilies which have very few annual species show a still smaller 

 percentage (three per cent) ; while those whose species are mostly 

 trees or shrubs have less than two per cent which extend to 

 more than two regions. 



He also finds that those with fleshy fruits have a wider dis- 

 persal than those with dry fruits, and those with very small 

 seeds, wider than those with larger seeds. Eighteen species 

 only are found to be spread over half the land surface of the 

 globe. There are no trees or shrubs among these; grasses are 

 most abundant among them ; and composites — the daisy and 

 aster family — the least ! This last conclusion seems very 

 strange in view of the fact that this family has its seeds so 

 frequently provided with special means of dispersal, either by 

 the wind or by animals. But he also points out, what is now 

 well known to botanists, that the species of Compositse are not 

 usually very widely spread ; and also that several other natural 

 orders in which the seeds are usually winged for wind-dispersal 

 are not more widely distributed 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 pur- 

 pose of securing ample means of reproduction Avithin 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 gen- 

 eral 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 study- 

 ing 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 consists in the 

 characterisation of the vegetation of each region or district by 

 the proportionate abundance in species belonging to the dif- 

 ferent natural orders. 



