BOTANICAL GEOGRAPHY. 419 



reflect on the well-known facts which Hiuds brings forward, without either 

 certainty or accuracy of detail, as the foundation of his opinions, they leave 

 room for the formation of other hypotheses besides his. His laws are as 

 follows : 1. In proportion as the districts of vegetation are further separated 

 from each other by the sea, so is the number of species of plants common to 

 them less. Hence the large number of species common to the three large 

 portions of the earth in the arctic zone, and hence the greater the contrast, 

 in comparing the floras of corresponding climates, the further south we pro- 

 ceed, the various portions of the earth in the southern hemisphere being 

 further separated from each other. 2. If we divide the whole earth into 

 six floral districts which would certainly be arbitrary enough we obtain 

 for each almost exclusively endemic species, and we may add, that the same 

 result would hold good were we to admit more than thirty districts. 3. In 

 different natural floras under corresponding climates, we certainly find 

 similar forms, but not the same species. 4. In some islands the vegetation 

 is entirely endemic ; these cannot, therefore, have obtained their plants by 

 migration from external sources, &c. All these and similar facts certainly 

 are opposed to the migration of plants from a single point of the earth's sur- 

 face to all the rest, and which scarcely any naturalist now believes ; but 

 there is a broad interval in the argument, which has not yet been filled up 

 with facts, between these considerations and the assertion, that centres of 

 creation do not exist, but that every point has produced the plants it pos- 

 sesses. We know that some regions of the earth contain many more endemic 

 species than others, without the soil or climate being sufficient to explain 

 this increase. The number of endemic forms diminish in the direction of 

 any climatal boundary, as it were, like the radii of a circle, in the centre of 

 which a centre of creation is situated ; hence, e. g. in Europe, we may speak 

 of western, eastern, or southern forms of plants, which gradually disappear, 

 one after the other, towards the east, the west, or the north. There does 

 not appear to be any other difference between an island which possessed 

 endemic plants only, as St. Helena, and a continental district, which, like 

 Spain or Illyria, abounds in endemic plants, than that other plants which 

 have migrated from external sources are associated with the latter, which 

 could not readily occur in the former case, on account of the distance of 

 any continent. On reviewing all the known facts, and seeking for the 

 simplest theory by which their relation may be explained, we are com- 

 pelled to adopt the supposition of the existence of as many centres of creation 

 as there are districts of endemic plants upon the globe. The difficulty of 

 determining in each case the original centre of creation, on account of the 

 mixture of the groups in the wide and connected districts of continents, is 

 so great, that it must always remain the main object of botanical geography. 

 It is only the problem of the groups of creation that gives this science a 

 peculiar importance, and raises it above the imputation of being an aggre- 



