THE THEORY OF EVOLUTION 



formations and in the Devonian 1 formation, widely 

 extended over the world it is true, but only in a few 

 and similar forms, while the Calamariaceae appear in 

 several families and individually in greater number, 

 we come near to the conclusion that under the luxurious 

 conditions of existence in the Carboniferous era 2 



a wide development 

 of the Calamariaceae 

 type took place in 

 various directions ac- 

 cording to the nature 

 of the habitat. That 

 would be a case of 

 differentiation of a 

 type. 



(6) 



The Fern 



Series. 



FIG. 20. a, SPHENOPTERIDIUM furcillatum ; 

 Silurian ; Heffen, Nassau. 6, S. dissectum ; 

 Kulm ; Rothwalthersdorf, Lower Silesia. 

 Primary venation. (After Gotlian.) 



The classification 

 of the fossil ferns is 

 effected for purely 

 practical reasons ac- 

 cording to the ' venation ' and according to the 

 mode of attachment of the pinnae (subdivisions) to 

 the stalk. The oldest ferns, the Archeeopteridse, show 

 a fan-like venation ; all the veins are of the same 

 thickness and radiate from about one point 3 (Fig. 20). 



1 According to Zittel: Handbuch der Paldontologie, V, p. 176. Gothan 

 puts a note of interrogation against Devonian. 



2 Potonie : Die Entstehung der Steinkohle, etc., p. 152. 



3 Gothan : Entwicklung der Pflanzemvelt, p. 21, 



