SUPPLEMENTARY SECTION. 



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time, any resemblances of facies that we may observe may have no con- 

 nection whatever vrith an unbroken continuity of specific types. 



I desire, however, under this head, to affirm my conviction that, with 

 reference to the Erian and Carbonifei-ous floras of North America and of 

 Europe, the doctrine of " homotaxis," as distinct from actual contem- 

 poraneity, has no place. The succession of formations in the Palaeozoic 

 period evidence? a similar series of physical phenomena on the grandest 

 scale throughout the northern hemisphere. The succession of marine 

 animals implies the continuity of the sea-bottoms on which they lived. The 

 head-quarters of the Erian flora in America and Europe must have been 

 in connected or adjoining areas in the North Atlantic. The similarity of 

 the Carboniferous flora on the two sides of the Atlantic, and the great 

 number of identical species, proves a still closer connection in that period. 

 These coincidences are too extensive and too frequently repeated to be the 

 result of any accident of similar sequence at diffcent times, and this more 

 especially as they extend to the more minute differences in the features of 

 each period, as, for instance, the floras of the Lower and Upper Devonian, 

 and of the Lower, Middle, and Upper Carboniferous. 



Another geographical question is that which relates to centres of dis- 

 persion. Li times of slow subsidence of extensive areas, the plants inhabit- 

 ing such areas must be narrowed in their range and often separated from 

 each other in detached spots, v/hile, at the same time, important climatal 

 changes must also occur. On the re- emergence of the land such of these 

 species as remained would again extend themselves over their former areas 

 of distribution, in so far as the new climatal and other conditions would 

 permit. We would naturally suppose that the first of the above processes 

 would tend to the elimination of varieties, the second, to their increase ; 

 but, on the other hand, the breaking up of a continental flora into that of 

 distinct islets, and the crowding together of many forms, might be a pro- 

 cess fertile in the production of some varieties if fatal to others. 



Farther, it is possible that these changes of subsidence may have some 

 connection with the introduction, as well as with the , extinction, even of 

 specific types. It is certain, at least, in the case of land plants, that such 

 types come in most abundantly immediately after elevation, though they 

 are most abundantly preserved in periods of slow subsidence. I do not 

 mean, however, that this connection is one of cause and effect ; there are, 

 indeed, indications that it is not so. One of these is, that m some cases 

 the enlargement of the area of the land seems to be as injarious to ter- 

 restrial species as its diminution. 



Applying the above considerations to the Erian and Carboniferouf 



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