president's address. 171 



whicli he carried on for many years, is worthy of notice, and of 

 being remembered by us. " It is not so much a total annual 

 average amount of heat that a plant requires to enable it to 

 vegetate, to flower, or to ripen its seed, as that this heat shall 

 never descend below, or ascend above certain extremes, and that 

 it shall remain within those limits for a sufficient length of time 

 for the completion of those operations, a period of time which 

 may be shortened or lengthened according to the greater or less 

 intensity of the heat received by the plant within the above 

 limits."* 



Again, it is worth pointing out that M. De Candolle seems to 

 " support the doctrine that sj)ecies of plants were successively 

 created f at different geological periods, and in different parts of 

 the earth ; that whilst some species have survived through seve- 

 ral geological periods, others have disappeared with the great 

 changes that have occurred in the configuration of the surface of 

 our globe." 



He tells us, " that of the species now existing, whilst the 

 great majority belong evidently to the earlier geological 

 periods, J there is reason to believe that the creation of others 

 dates only from the epoch of those phenomena which produced 

 the present geographical conformation. But there is no evidence, 

 nor any plausible ground, to suppose that any species has been 

 added to the Vegetable Kingdom since the creation of man. On 

 the other hand, it is well known that within our historic times 

 certain species of plants have been gradually restricted in their 

 area, and have even finally disappeared, either from natural 

 causes depending on Geolofjical changes, or by the direct or indi- 

 rect agency of man." 



M. de Candolle, in thus stating the necessity for local Floras, 

 " La Botanique G6ographique demande des Flores Locales, 

 completes," further remarks: " J'ai 6te surpris de ne trouver 



* See " Edinburgh Review," No. 212, for October, 185G. 



t His words are, " II y a plusieurs motifs G6ologiques et Botaniques pour croire ^ 

 une apparition successive des Esp6ces." — Vol. ii., p. iv. 



J De Candolle observes (p. 1059), '''Les Esp6ces qui composent actuellement le rSgne 

 Vfiggtal, ou du moins la majority d' entre elles, paraissent remonter a un temps recuIC-, 

 anterieur a plusieurs des faits aetuels G6o^raphiques et pliysiques." 



VOL. III. PT. HI. W 



