HORSETAILS 211 



such as complexity of the leaf, the earliest known 

 members were the most elaborate; reduction to 

 greater simplicity in such points is, however, 

 quite consistent with a general advance, and it is 

 probable that the highest level of organisation 

 was reached by the Calamites of the later Palaeo- 

 zoic times towards the close of the Carboniferous 

 epoch. Species from the Upper Coal Measures 

 certainly show great elaboration, both in the 

 fructification and in the structure of the wood. 

 From that time onwards the downward course 

 began, and its stages can be traced right through 

 the geological record of Mesozoic and Tertiary 

 times, down to our own day. Such records are 

 always imperfect, but the evolution or degeneration 

 of the Horsetail stock can be followed more con- 

 tinuously than that of the Club-mosses. Certain 

 lines must no doubt have died out altogether; 

 for example, the heterosporous Calamites seem to 

 have left no descendants, but still there are good 

 grounds for believing that our existing Horsetails 

 may claim as their ancestors some of the gigantic 

 Calamites of the Palaeozoic period. 



Our records do not go far enough back to throw 

 any light on the origin of the class Equisetales, 

 but we shall understand their position better after 

 considering another group, wholly fossil, which 

 has some characteristics in common with them, 

 though very distinct in other ways. This is the 



