112 THE CHAIN OF LIFE. 



shrubs of temperate climates, or in the Palms and their allies, 

 which figure so conspicuously within the tropics. The few rare, 

 and to some extent doubtful, representatives of these types 

 scarcely deserve to be noted as exceptions. Had a botanist 

 searched the Palaeozoic forests for precursors of the future, he 

 would probably have found only a few rare species, while he 

 would have seen all around him the giant forms and peculiar 

 and monotonous foliage of tribes now degraded in magnitude 

 and structure, and of small account in the system of nature. 



It must not be supposed that the Palaeozoic flora remained in 

 undisturbed possession of the continents during the whole of 

 that long period. In the successive subsidences of the conti- 

 nental plateaux, in which the marine limestones were de- 

 posited, it was to a great extent swept away, or was restricted 

 to limited insular areas, and these more especially in the far 

 north, so that on re-elevaticn of the land it was always peopled 

 with northern plants. Thus there were alternate restrictions 

 and expansions of vegetation, and the latter were always 

 signalised by the introduction of new species, for here, as 

 elsewhere, it was not struggle, but opportunity, that favoured 

 improvement. 



In the Lower Silurian such lants as existed must have ex- 

 perienced great restriction at the age of the Niagara or Wen- 

 lock limestone. Those of the Upper Silurian suffered a similar 

 reverse at the time of the Lower Hederberg or Ludlow lime- 

 stones. This recurred at the close of the Devonian and in 

 the time of the Lower Carboniferous limestone ; and finally the 

 Palaeozoic flora disappeared altogether in the Permian, to be 

 replaced by new types in the Mesozoic, While, therefore, there 

 is a great general similarity in the successive Palaeozoic floras, 

 there are minor differences, so that the Devonian plants are for 

 the most part distinct specifically from those of the Lower Car- 

 boniferous, those of the Lower Carboniferous from those of 

 the Coal-formation, and those of the latter from those of the 

 Permian. 



