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SUPPLEMENTARY SECTION. 7 



of one or other of their constituent elements. In this way such plants as 

 Arthrostigma and Fsilophyton may have assumed new varietal forms, 

 (2) By embryonic retardation or acceleration,* whereby certain species 

 may have had their maturity advanced or postponed, thus giving them 

 various grades of perfection in reproduction and complexity of structure. 

 The fact that so many Brian and Carboniferous plants seem to be on the 

 confines of the groups of Acrogcns and Gymnosperms may bo supposed 

 favourable to such exchanges. (3) The contraction and breaking up of 

 floras, as occurred in the Middle Erian and Lower Carboniferous, 

 may have been eminently favourable to the production of such varietal 

 forms as would result from what has been called the " struggle for exist- 

 ence," (4) The elevation of a great expanse of new land at the close of 

 the Middle Erian and the beginning of the Coal period, would, by per- 

 mitting the extension of species over wide areas and fertile soils, and by 

 removing the pressure previously existing, bo eminently favourable to the 

 production of new, and especially of improved, varieties. 



2. Whatever importance we may attach to the above supposed causes 

 of chaise, we still require cu account for the origin of our specific types. 

 This may forever elude our observation, but we may at least hope to ascer- 

 tain the external conditions favourable to their production. In order to 

 attain even to this it will be necessary to inquire critically, with reference 

 to every acknowledged species, what its claims to distinctness are, so that 

 we may be enabled to distinguish specific types from mere varieties. 

 Having attained to some certainty in this, we may be prepared to inquire 

 whether the conditions favourable to the appearance of new varieties were 

 also those favourable to the creation of new types, or the reverse — whether 

 these conditions were those of compression or expansion, or to what extent 

 the appearance of new types may be independent of any external condi- 

 tions, other than those absolutely necessary for their existence, I am not 

 without hope that the further study of fossil plants may enable us thus to 

 approach to a comprehension of the laws of the creation, as distinguished 

 from those of the continued existence of species. 



In the present state of our knowledge we have no good ground either 

 to limit the number of specific types beyond what a fair study of our mate- 

 rial may warrant, or to infer that such primitive types must necessarily 

 have been of low grade, or that progress in varietal forms has always been 

 upward. The occurrence of such an advanced and specialized type as that 

 of Syringoxylon, in the Middle Devonian, should guard us against these 

 errors. The creative process may have been applicable to the highest as 

 well as to the lowest forms, and subsequent deviations must havQ included 



• In the manner illustrated by Hyatt and Cope. 



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