H. BOTANY. 367 



herbaceous perennials, and by the absence of members of the 

 preceding groups. The next group (Hekistotherms) com- 

 prises arctic or alpine plants, requiring very little heat, and 

 able to endure the long darkness of winter. The orders rep- 

 resented are not numerous, and woody species are compara- 

 tively rare. A sixth group, styled Megistotherms, may be 

 formed of such plants as demand an extreme degree of heat, 

 now scarcely represented upon the earth, but formerly includ- 

 ing the algae, ferns, and lycopods of the carboniferous period. 

 The constitutional peculiarities of plants upon which this 

 grouping is based are accounted for by no known modifica- 

 tions of either external or internal structure or composition. 

 L>e Candolle attempts to explain them upon the principle of 

 heredity, the present constitution being the result of long- 

 continued habit. He considers it an established fact that in 

 the earliest ages there prevailed over the entire globe a uni- 

 form climate of high temperature, followed by a very gradual 

 cooling, and the establishment by slow degrees of distinct 

 and localized climates. He assumes, as a necessary conse- 

 quence, that the earliest flora must have been as general and 

 as uniform, consisting wholly of Megistotherms, which, as the 

 temperature declined, were gradually driven from the poles 

 toward the equator, their place becoming occupied by the 

 other groups, successively, or as circumstances favored. In 

 Avhat way the several groups were developed is a question 

 kindred to that respecting the evolution of specific forms, 

 and the hypothesis of Darwin is equally applicable in its 

 solution. The correctness of that hypothesis De Candolle 

 does not consider, but he expresses the opinion that it is the 

 only hypothesis worthy of discussion that has yet been ad- 

 vanced. 



GEOGRAPHICAL DISTRIBUTION OF THE CUPULIFER^E. 



This large family, including the chestnuts, oaks, and beech- 

 es, since it is the earliest geologically of the dicotyledonous 

 plants, affords much promise of definite conclusions in regard 

 to the genetic relations of the present species from a com- 

 parison of the living and fossil forms, and the first results of 

 an extended investigation in this direction by A. S. Oersted, 

 in which the morphology, classification, and geographical 

 distribution of the family are treated, has been published in 



