4 PLANT GEOGRAPHY 



causes which act upon plants, and the response that the 

 plants make in function and structure to such causes, 

 so botanical geography has to do both with the causes 

 and the results of the distribution of plants. Such a 

 study has, as has been suggested, its interest for the 

 artist, the traveller, the scientific geographer, the man 

 of commerce interested in the sources of industrial 

 produce, the horticulturist, and the botanist. As a 

 department of scientific inquiry, however, its main 

 importance is biological. It is part of the more general 

 science of the distribution of living beings ; and, to some 

 extent, the laws governing the distribution of plants 

 apply also to that of animals. It is these general 

 principles that are of the greatest interest to the student 

 of science. The way in which one or more species of 

 plants have reached a particular area is more important 

 to him than the fact that they have done so, far more 

 important than the economic uses to which they may 

 be put. Treating the subject as a branch of natural 

 history, he may almost ignore those changes in distribu- 

 tion that man has intentionally produced, the introduc- 

 tion of agricultural plants into new lands, and the 

 appropriation of huge areas for the cultivation of a few 

 valued species, such as the cereals, the pasture grasses, 

 cotton and other textile substances, etc. Man's in- 

 voluntary dispersive action, on the other hand, must 

 be taken into consideration along with that of other 

 animals. 



Whilst it is obvious that the present distribution of 

 land and water over the surface of the globe must 

 profoundly affect the areas occupied by land plants 

 and by marine algae, as well as the present distribution 

 of temperature, rainfall, etc., a slight knowledge of 

 geology will enable us to appreciate the fact that this 

 distribution of land and water has not always been the 

 same. Any description of the present distribution of 

 plants, dependent as it must be upon present geogra- 

 phical conditions, can, only be temporarily true. These 

 geographical conditions and their biological results have 

 not always been the same ; nor will they remain as they 

 are. The " natural floras," or assemblages of associated 

 plants, of to-day are only of to-day, and we must not 

 ignore the influence of former conditions and changes 

 in the distribution of land and water. Fragmentary 



