GENERAL LAWS OF ORIGIN AND MIGRATION. 261 



likely that we should have failed to discover their con- 

 tinuance, we may fairly assume that they have become 

 extinct, at least locally ; and where the field of observa- 

 tion is very extensive, as in the great coal-fields of Europe 

 and America, we may esteem such extinction as practi- 

 cally general, at least for the northern hemisphere. 

 When many specific types become extinct together, or in 

 close succession, we may suppose that such extinction 

 resulted from physical changes ; but where single types 

 disappear, under circumstances in which others of similar 

 habit continue, we may not unreasonably conjecture that, 

 as Pictet has argued in the case of animals, such types 

 may have been in their own nature limited in duration, 

 and may have died out without any external cause. 



6. With regard to the introduction of specific types 

 we have not as yet a sufficient amount of information. 

 Even if we freely admit that ordinary specific forms, as 

 well as mere varieties, may result from derivation, this by 

 no means excludes the idea of primitive specific types 

 originating in some other way. Just as the chemist, after 

 analysing all compounds and ascertaining all allotropic 

 forms, arrives at length at certain elements not mutually 

 transmutable or derivable, so the botanist and zoologist 

 must expect sooner or later to arrive at elementary 

 specific types, which, if to be accounted for at all, must 

 be explained on some principle distinct from that of 

 derivation. The position of many modern biologists, in 

 presence of this question, may be logically the same with 

 that of the ancient alchemists with reference to the 

 chemical elements, though the fallacy in the case of fos- 

 sils may be of more difficult detection. Our business at 

 present, in the prosecution of paleobotany, is to discover, 

 if possible, what are elementary or original types, and, hav- 

 ing found these, to enquire as to the law of their creation. 



7. In prosecuting such questions geographical rela- 

 tions must be carefully considered. When the floras of 



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