258 THE GEOLOGICAL HISTORY OF PLANTS. 



nature. Even then we shall perhaps be able to compre- 

 hend only the means by which, after specific types have 

 been created, they may, by the culture of their Maker, 

 be "sported" into new varieties or subspecies, and thus 

 fitted to exist under different conditions or to occupy 

 higher places in the economy of nature. 



Before venturing on such extreme speculations as 

 some now current on questions of this kind, we would 

 require to know the successive extinct floras as perfectly 

 as those of the modern world, and to be able to ascertain 

 to what extent each species can change either spontane- 

 ously or under the influence of struggle for existence or 

 expansion under favourable conditions, and under arctic 

 ■semi-annual days and nights, or the shorter days of the 

 tropics. Such knowledge, if ever acquired, it may take 

 ages of investigation to accumulate. 



As to the origin and mode of introduction of succes- 

 sive floras, I am, for the reasons above stated, not disposed 

 to dogmatise, or to adopt as final any existing theory of 

 the development of the vegetable kingdom. Still, some 

 laws regulating the progress of vegetable life may be 

 recognised, and I propose to state these in connection 

 with the Palseozoic floras, to which my own studies have 

 chiefly related. 



Fossil plants are almost proverbially uncertain with 

 reference to their accurate determination, and have been 

 regarded as of comparatively little utility in the decision 

 of general questions of palaeontology. This results prin- 

 cipally from the fragmentary condition in which they 

 have been studied, and from the fact that fragments of 

 animal structures are more definite and instructive than 

 corresponding portions of plants. 



It is to be observed, however, that our knowledge of 

 fossil plants becomes accurate in proportion to the extent 

 to which we can carry the study of specimens in the beds 

 in which they are preserved, so as to examine more per- 



