INFLUENCE OF LAWS OF DEVELOPMENT. 665 



Palmetto in North America. The Heaths (Erica} of Europe are repre- 

 sented by different species at the Cape of Good Hope. The East and 

 West Indies and Africa have their peculiar Palrnaceae, Zingiberaceaa, 

 Marantacese, &c. The succulent Cactacese of Central America represent 

 the fleshy Euphorbiacese, Asclepiadaceae, and Mesembryanthacese of 

 Africa. The Cape Heaths are represented in Australia by Epacridacese, 

 associated in like manner with Myrtacese and Proteacese. The Firs of 

 the southern hemisphere belong to genera distinct from those of the 

 northern hemisphere, &c. 



Specific Centres. The evident absence of a constant relation 

 between the existence of certain climatal conditions and the occur- 

 rence of particular species of plants stil] more, the existence of 

 limited areas of distribution, often exhibiting centres of greatest 

 abundance have led to the supposition that the individual species 

 of plants have been created at particular centres, whence they 

 have spread themselves over more or less extensive tracts, in the 

 course of their extension becoming intermixed with other species, 

 and thus producing complex assemblages. 



The spreading of what are called social plants, such as Pasture-Grasses, 

 Furze, various forest-trees, &c., illustrates the facility w r ith which many 

 plants extend themselves over new ground. The Anacharis, a North- 

 American water-plant, has been diffused all over England within the last 

 few years. The diffusion of certain plants is also greatly dependent on 

 the success of some plants over others in the struggle for existence which 

 is ever going on in nature. It does not follow that the wild plants in any 

 given area are those which are best adapted for that situation. They are 

 often the weakest, and are overpowered by other plants. In this way 

 native vegetation constantly gets exterminated by foreign intruders. 



In this hypothesis it is unimportant whether we imagine a 

 single plant (or pair of dioscious plants) or a more or less extensive 

 assemblage of individuals to have been created on the same spot. 

 This is a question impossible to be solved by science. 



Some authors believe that species have been created at many points 

 where the conditions were fitting, explaining in this way the interrupted 

 areas of certain plants. 



Successive Creation of Species, The facts revealed by Geology, 

 as well as by Botany itself, tend to prove that the creation of 

 species has not been simultaneous, but successive, in different 

 geological periods. Not only do we find in the* older forma- 

 tions fossil plants different from living vegetation, but those beds 

 immediately preceding the present surface of the earth contain not 

 only remains of animals of existing species, but fossil plants closely 

 resembling, if not identical with, existing plants. 



Existing Agencies not sufiicient to account for Plant-distri- 

 bution. Numerous cases of scattered distribution of existing 



