70 REVIEWS. 



which abound most in species ; and Chapter xxiii., as regards 

 their most characteristic natural families. Chapter xxiv., on 

 the variety of vegetable forms in different countries and in the 

 world at large, i. e., the probable number of species, the pro- 

 portion of genera to species, and of orders of genera and 

 species. Chapter xxv., the division of the earth's surface 

 into natural botanical regions. Chapter xxvi., sketch of the 

 vegetation of the different countries in respect to the probable 

 origin of their existing species, etc. 



The Fourth Book, of a single brief chapter, consists merely 

 of a summary of the author's general conclusions. We give 

 these entire, for convenience availing ourselves of a translation 

 in Hooker's " Journal of Botany." 



" The plants now inhabiting the globe have survived many 

 changes, geological, geographical, and, latterly, historical. 

 The history of their distribution is hence intimately connected 

 with that of the whole vegetable kingdom. 



" To explain existing facts, it is fortunately unnecessary to 

 adopt any conclusion upon the most obscure hypotheses of 

 cosmogony and palaeontology, or on the mode of creation of 

 species, the number originally created, and their primitive 

 distribution. Botanical geography can indicate certain prob- 

 abilities, certain theories, but the principal facts in distribu- 

 tion depend upon more recent and less obscure causes. It 

 suffices to understand and to allow certain facts and theories, 

 which appear probable, namely, that groups of organized be- 

 ings under different hereditary forms (classes, orders, genera, 

 species, and races) have appeared in different places and at 

 different times ; the more simple perhaps at first, the more 

 complicated afterwards ; that each of these groups has had a 

 primitive centre of creation of greater or less extent ; that 

 they have, during the period of their existence, been able to 

 become more rare or common, to spread more or less widely, 

 according to the nature of the plants composing them, the 

 means of propagation and diffusion they are possessed of, the 

 absence or presence of animals noxious to them, the form and 

 extent of the area they inhabit, the nature of the successive 

 climates of each country, and the means of transport that the 



