138 EVOLUTION OF PLANTS 



transitional forms, like Osmunda, which connect per- 

 fectly the two groups. We should naturally expect that 

 the most specialized forms, i.e. those which have diverged 

 most widely from the primitive stock, would be the pre- 

 vailing forms at the present time, and such really is the 

 case. The Polypodiacese, which include all the com- 

 moner ferns and are with little question the most 

 specialized of the ferns, far outnumber all the other 

 families combined, and are preeminently the modern 

 type. 



It is interesting to note that the conclusions reached 

 by a study of comparative morphology are confirmed by 

 the geological record. The oldest ferns known are be- 

 yond question Eusporangiates, all of the ferns found 

 in the Carboniferous and pre-Carboniferous rocks prob- 

 ably being of this character, while undoubted Lepto- 

 sporangiates first appear in the Mesozoic formations, 

 from which time they appear to have increased in num- 

 ber and variety, gradually replacing the eusporangiate 

 ferns of the earlier formations. There is no evidence 

 that the Leptosporangiates have ever been any more 

 abundant than at the present time, and they are prob- 

 ably to be considered as a distinctly modern type. 



