188 EVOLUTION OF PLANTS 
transitional forrns, like Osmunda, which connect per- 
fectly the two groups. We should naturally expect that 
the most specialized forms, z.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 Polypodiacezw, 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 preéminently 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. 
