136 EVOLUTION OF PLANTS 
sporangiates, in spite of their numbers, might all be 
included in a single order, Filices, were it not for 
a small number of the heterosporous forms, #.e., those 
having two kinds of spores, which are, however, evi- 
dently related to the Filices. A study of the different 
orders of the Eusporangiate indicates that we have to 
do with remnants of once much larger groups, of which 
most of the members have become extinct. The much 
greater homogeneity of the Leptosporangiate, as well 
as their numbers, indicate on the other hand a special- 
ized and presumably more modern type of vegetation, 
and this is borne out by a study of their distribution. 
None of the Eusporangiate ever occur in any great 
numbers together, although some of them are cosmo- 
politan. One order, the Marattiacez, are strictly tropi- 
cal plants, and usually occur as isolated individuals or 
in small groups. Among the Leptosporangiates, on 
the other hand, the plants are often gregarious, and 
form conspicuous features of the vegetation. The com- 
mon brake, Pteris aquilina, and in the tropics, species of 
Gleichenia, form tangled thickets and cover extensive 
tracts almost to the exclusion of other vegetation. If 
we analyze the fern-flora of those tropical regions 
where ferns form an important feature of the vege- 
tation, the disproportion in numbers between the eu- 
sporangiate and leptosporangiate species is even greater 
than in temperate regions. Thus in Jamaica, which is 
exceptionally rich in ferns, out of about five hundred 
species described from the island, less than a dozen are 
eusporangiate, and of these none are common enough 
to make any impression upon the general character of 
the vegetation, although an occasional gigantic Marattia 
