272 KEY TO TERRESTRIAL HISTORY 



probable. If we turn our attention to the closing days of 

 the Palaeozoic period, we find that two very different 

 floras were then in existence : one occurring in the 

 northern hemisphere, characterised by Lepidodendra and 

 Sigillaria, the other in the southern hemisphere con- 

 taining much more highly organised plants, such as 

 Conifers, and characterised by abundant ferns, particularly 

 Glossopteris and Gangamopteris. Thus Australia, which 

 is now behind the times as regards its indigenous inhabi- 

 tants, peopled by an old-fashioned flora and fauna, was 

 then as far ahead, compared with our northern lands : it 

 led the way, and had already given birth to a Mesozoic 

 flora. The state of affairs imagined by Huxley would 

 seem at this time to have been partly realised since a 

 Mesozoic flora in the southern was flourishing contem- 

 poraneously with a Palaeozoic flora in the northern 

 hemisphere. It might be supposed that all the conditions 

 requisite for an inversion in the order of succession of our 

 fossil floras were now fulfilled. Like tw T o opposing armies, 

 the two hosts of plants stood facing each other all round 

 the world, with the equator for their frontier. If any- 

 where the northern forms should succeed in effecting a 

 crossing, and obtaining a foothold in the south, the fact 

 would be recorded by an inversion of the succession ; 

 Palaeozoic plants would be found in deposits lying upon 

 others containing Mesozoic species (Fig. 85). 



Such an inversion has never been brought to light, 

 and thus the absence of inversion does not prove the 

 impossibility of the simultaneous coexistence of two 

 different floras. 



Improbable as we have supposed it to be, migration 

 was, so far as we know, everywhere in one direction, in 

 the direction, that is, of a yielding foe. The newer flora 

 of the south maintained a steady advance northwards, the 

 older flora of the north everywhere gave way before it, 



