others existed, v*^liose generic representatives are now 

 peculiar to Southern Africa. This tendency to extinc- 

 tion cannot be attributable to a decrease of temperature 

 like that of the Palm tribe and other tropical plants, 

 nor to the introduction of more vigorous plants ; but to 

 a slow and inevitable decUne, due, probably, to their 

 unyielding character, as well to a difficulty of propa- 

 gation. Their decline, which was accompanied with 

 the appearance of some of the first dicotyledonous 

 angiosperms, was so effectual that for ages more or less 

 remote, they have left no trace in Europe. The living 

 representatives of cycads are dispersed over the globe, 

 but only in small groups, separated by enormous tracts 

 and confined to the tropical and temperate regions of 

 Asia, Southern Africa, America, and Austraha. They 

 prefer the slopes of mountains and moist sandy spots 

 sheltered by trees, for the protection and growth of the 

 young plants, whose stages to maturity are slow and 

 lingering. The internal structure of the whole family 

 is similar, consisting of a large pith, the cells of which 

 afibrd the tree a rich supply of starch. The stem is 

 very slow of growth, and becomes in course of years a 

 stout column, which sometimes attains a height of some 

 feet. The surface of the stem is furnished with leaves, 

 arranged spirally, and a rosette of large foliage leaves 

 is annually or biennially produced, in the centre of 

 which the terminal bud is embosomed, enveloped 

 with scales, under whose protection the new whorl of 

 leaves is slowly formed. An axillary bud is often 

 attached to the exterior of the trunks of fossil cycads, 



