SUPPLEMENTARY SECTION. 8 



new types appear in the poorer period this conclusion seems less probable. 

 For example, new types appearing in poor formations, like the Lower 

 Erian and Lower Carboniferous, have greater significance than if they 

 appeared in the Middle Erian or in the Coal Measures. 



(5.) When specific types disappear without any known successors, 

 under circumstances in which it seems unlikely that we should have failed 

 to discover their continuance, we may fairly assume that they have become 

 extinct, at least locally ; and Avhere the field of observation is very exten- 

 sive, as in the great coal fields of Europe and America, we may esteem 

 BucY extinction as practically 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 suflScient amount of information. Even if we freely admit that ordi- 

 nary specific forms, as well as more 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 analyzing 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 fossils may be of more difficult detection. Our business at 

 present, in the prosecution of palaeobotany, is to discover, if possible, what 

 are elementary or original types, and, having found these, to enquire as 

 to the law of their creation. 



(7.) In prosecuting such questions geographical relations must be care- 

 fully considered. When the floras of two successive periods have existed 

 in the same region, and under circumstances that render it probable that 

 plants have continued to grow on the same or adjoining areas throughout 

 these periods, the comparison becomes direct, and this is the case with the 

 Erian and Carboniferous floras in North-Eastern America. But when the 

 areas of the two formations are widely separated in space, as well as in 



