70 



THE THEORY OF EVOLUTION 



Since now chronologically also, from the Silurian 

 to the upper productive coal measures, the said evolu- 

 tionary forms follow each other, therefore the Ferns 

 appear to present an evolutionary series and the 

 preliminary of the transformation itself to have been 

 a differentiation of the original Archseopterid type. 

 The external cause we cannot discuss, since with the 

 palseontological evidence we have only to compare 

 the results of vital processes with each other. 



-:g 



I 





FIG. 23. a, &IGILLAJUA Brardi (leiodermic) from the upper productive Car- 

 boniferous formation of Wettin. b, 8. Blob ay (rhytidolepic) from the 

 middle productive Carboniferous formation of the Ruhr district, c, S. 

 elegant ula (' favularic,' ex same), d, SYRJXGODENDROX (state of preserva- 

 tion), c and d show how different states of preservation occur by removal 

 of separate bark layers. (After Got ha n.) 



(c) The Sigillaria Series. 



A similar transformation, though caused by changed 

 external conditions; we can follow up with the fossil 

 Sigillaria, 



According to the arrangements of the leaf scars 

 the Sigillaria have been classified as ' favularic ' when 

 they stand quite close together in sloping lines, 

 ' rhytidolepic ' when they appear in longitudinal rows 

 with furrows between, ' leiodermic ' where the scars 

 stand quite separated on the bark without longitudinal 



