INVASION BY THE FITTEST 273 



and the triumphant progress of the Mesozoic forests did 

 not cease till they had dominated the world. 



Here, then, we may expect to find the clue to the un- 

 failing regularity with which systems of organisms 

 succeed each other all over the world : inversion does not 

 occur because the newer forms of life, in virtue of their 

 higher organisation, are more efficient than the old. 



Although the instance we have just considered is the 



FIG. 85. The Northern and Southern Floras at the close of the 

 Carboniferous Epoch. The northern flora is indicated by 

 black patches, the southern by dots. After Seward. 



most striking to be found, it is certainly not so extreme 

 as that conjectured by Huxley, and on the whole, so far 

 from diminishing our faith in William Smith's method, is 

 rather likely to strengthen it, since it plainly appears that 

 when two different floras do happen to coexist, this 

 method is not incompetent to discover the fact. 



We may now resume our account of the progress of 

 discovery, as geologists continued to identify strata by 

 fossil remains. In William Smith's time and that of his 



19 



