GEOGRAPHIC AFFINITIES 
231 
localized from northern Brazil to Vera Cruz and by way of the Antilles 
and the coastal plain extending to eastern Massachusetts. 
These austral groups, Schizaea, the Xyridaceae and Haemadora- 
ceae, are merely illustrative cases of a large series of families and genera, 
which in temperate North America are confined to a very restricted 
region of the Atlantic slope. Other genera, widely dispersed in the 
southern hemisphere and the tropics but essentially unknown in 
continental Eurasia, are more generally dispersed in North America. 
Here belongs the xerophytic genus Pellaea of southern and eastern 
Africa, the Cape Verde Islands, the Azores, India, Flores Island, 
Australia, Tasmania, New Zealand, various Pacific Islands, the Andes, 
and mountains of southeastern Brazil; and in North America widely 
dispersed from Costa Rica to British Columbia, Mackenzie and wes- 
tern New England. As notable as any species is the extremely xero- 
phytic P. densa, a unique species, with a known occurrence in Costa 
Rica, arid mountains from California to southern British Columbia 
and Idaho, locally in the central Rocky Mountains, the Bruce Penin- 
sula in Ontario, and arid mountain-walls of Megantic and Gaspe Cos., 
Quebec. Here, then, is a species of a widely dispersed austral genus 
highly developed in the Sierra Nevada and Cascade Mountains but 
locally abundant at remote points quite to the eastern margin of the 
continent. Another illustration inevitably suggested by Pellaea 
densa is that remarkable group of xerophytic ferns constituting a well- 
marked section or subgenus of Polystichum. I refer to P. mohrioides 
and its allies (fig. 17). There are four or five species of this alliance, 
all plants of the highest degree of localization. P. mohrioides and 
other austral species are known only from the Antarctic Prince Edward 
Islands, 1,200 miles southeast of the Cape of Good Hope, from the 
Falkland Islands, Tierra del Fuego, and Patagonia, and as the rarest of 
isolated species in the Andes. In North America we have two species 
so close to P. mohrioides that some authors have considered them 
inseparable: P. Lemmoni, a famous rare species of the mountains of 
Cahfornia, Oregon and Washington; and P. scopulinum of similar 
range, though even rarer, and found with Pellaea densa on arid moun- 
tain-walls of Gaspe County, Quebec. 
This Fuegian affinity is not confined, however, to the extreme xero- 
phytes. It occasionally appears in pronounced hydrophytes. For in- 
stance, the plant of wet subsaline shores from the Mississippi Valley to 
the Pacific which has erroneously passed as Rumex persicarioides has been 
