430 



Scientific Intelligence. 



happy distinction between the temperatures actually operative in vegeta- 

 tion, and those which (being below the freezing point, &c.) are altogether 

 null for vegetation, and ought to be eliminated from the tables of mean 

 temperature, when these are viewed in relation to the Northern and 

 Southern geographical range of species. 



In Book 2, Geographical Botany, or the study of species, genera, and 

 families, from a geographical point of view ; Chap. 4 relates to the limi- 

 tation of species upon plains and upon mountains, and the probable causes 

 of their actual limits, applied both to spontaneous and cultivated plants ; 

 and there is a good endeavor to show that the Northern limit of species 

 is fixed rather by the sum of heat available for vegetation during the 

 growing season, than by the mean temperature of the year. Chap. 5 

 treats of the shape of the area occupied by a species, a very curious point ; 

 and it seems that the area of species inclines to be circular or elliptical. 

 Chap. 6 treats of the association or disjunction of the individuals of a 

 species in its area. Chap. 7 treats of the area of species as to extent of 

 surface, considered as to the families they belong to, as to stations, as to 

 size and duration of the plant, and as to the character of the fruit and 

 seed, whether affording facilities to dispersion or not. Chap. 8 considers 

 the changes which may have taken place in the habitation of species, 

 and discusses with great fullness the whole subject of naturalization, the 

 obstacles in the way, the causes and means of transport, and the inter- 

 changes which have been effected between the New and the Old Worlds. 

 Chap. 9 is a very long and interesting one, on the geographical origin of 

 the principal cultivated plants, not only those intentionally, but also those 

 unintentionally cultivated by man, — a chapter full of valuable matter, 

 carefully collected and well discussed.* Chap. 10 treats of disjoined spe- 

 cies, — those occupying two or more widely separated areas, and not in in- 

 termediate stations. Chap. 11 discourses of the early condition and proba- 

 ble origin of the existing species ; and brings out the various facts which 

 go far to prove the geological antiquity of the greater part of existing 

 species ; and that their creation was probably successive. Chap. 12 treats 

 of genera, and their geographical distribution, and maintains the view, 

 (in which we by no means coincide) that genera are truly naturally-limited 

 groups, even more so than species. Chap. 13 is devoted to the distribu- 

 tion of the species of a genus within its area. Chap. 14 treats of the 

 extent of surface occupied by genera. Chap. 15 discourses of the origin 

 and duration of genera. Chap. 1 6-1 9 treat of families, as to their area, 

 geographical limits, the distribution of species within the area of the 

 family, &c. 



The Third Book is devoted to Geographical Botany, or the characters 

 of different countries considered as to their vegetation. Chap. 20, of the 

 characters of the vegetation of a country; considered, in Chap. 21, as to 

 the relative numbers in the great classes respectively. Chap. 22, com- 



* It is singular that M. De Candolle should be so slow to abandon the idea that the 

 aborigines of Carolina, or any other part of North America cultivated or knew any- 

 thing of the Potato, which, if Raleigh obtained them in Carolina, were certainly im- 

 ported thither. But, though our aborigines had no potatoes, they had pumpkins or 

 squashes and beans, which all writers upon the history of cultivated plants have 

 overlooked, except the late Dr. Harris. 



