NOTICES OF BOOKS. 117 



wider range in America; and when the oceanic currents between Canada 

 and Scotland were stronger and more direct. 



Under the head of tropical disconnected (dlsjoinles) species a list 

 of a considerable mimber is given, with the caution that many may be 

 considered as naturalized. About 50 however remain as most unlikely 

 to have been transported by causes now in operation, and the majority, 

 without being truly aquatic, inhabit moist places. It appears proved to 

 M. de Candolle that the " disconnected species*' are much less numerous 

 in the tropics than in the northern extra-tropical regions ; * and upon 

 the whole the study of the tropical species leads M. de Candolle to 

 the same general conclusion as that of the extra-tropical did, viz. that 

 in the present state of our knowledge there appears to be no direct so- 

 lution of the problem of their dispersion. 



Cliapier 11. On the early condition and probable origin of existing 

 . Species in a state of Nature. — Under tlie first division of this subject 

 M. de Candolle declares the impossibility of explaining many facts, 

 from a study of the species themselves in relation to the existing con- 

 dition of things on the earth's surface. He observes that in numerous 

 cases he has been obliged to own that existing causes are insufficient 

 to explain well-established facts in distribution, and that the true but 

 anterior causes must be sought in difiercnt conditions of different areas 

 on the globe, in different states of species, or in a different distribu- 

 tion of land and sea, in changes of climate, and in different means 

 of transport. 



The facts that have led to this conclusion are : — 



1. Certain species are wanting in regions so well adapted to their 

 existence, that when once artificially introduced there, they forthwith 

 establish themselves like natives of the country. 



2. Woody plants flourish in mass in countries where the same spe- 

 cies cannot re-establish themselves after they have been once removed. 



3. Species with large seeds grow in countries between which there 

 are insuperable obstacles to their seeds having been transported. 



4. Many species are common to the tops of very distant mountains, 

 between which an interchange of seeds seems to be inconceivable. 



5. With regard to widely distributed aquatic, etc. plants, maiiy 



* A co.Lclusic>u ll.e accuracy of wliich we extrt'inely Joubt fur reasons slatcJ ii. a 

 note to pa-e 62. TJiat there sue Lowcvtr fewer discoiinccte.l si>eiie» iii proportiou 

 to the whote tropical than temperate flora, is most hkcly the case. 



