Biological Succession in Aleutians — Bank 
boreal regions. Natural ranges of hundreds 
of plants were mapped. Ranges which were 
more or less congruous were grouped. Each 
group was thus made up of superimposed 
ranges which were approximately equiformal 
and of progressively larger area. Hulten termed 
these "equiformal progressive areas.” He 
showed that each such area has a center, i.e., 
a region where the largest number of bio- 
types occurs. This is presumed to be the 
center of origin, and from here the plants 
dispersed to occupy their present ranges. 
Those with ranges that approximate closest 
the center are termed "centrants” and those 
with wider ranges are termed "radiants.” 
Geographically, the centers all lie outside the 
maximum advance of the ice sheets and thus 
probably served as the major plant refuges 
during Pleistocene glaciation. Some plants 
which survived in the more extensive refugia 
retained their genetical ability to reproduce 
and quickly occupy new ice-free areas follow- 
ing the glacial periods. Others, on more iso- 
lated, smaller refugia, became depauperated 
of biotypes with the result that their spreading 
ability was reduced. Hulten has termed them 
"rigid” species. The quickly spreading plants 
are termed "plastic” species. 
One of the most important plant refuges 
was an ice-free lowland which Hulten has 
called "Beringia,” a land area thought to have 
existed at various times during and imme- 
diately following the Pleistocene when suf- 
ficient water was withdrawn from the ocean 
to expose much of the sea floor in the north- 
ern and eastern Bering Sea and southern 
Arctic Ocean regions (Fig. 1). From Ber- 
ingia, from unglaciated portions of the eastern 
Aleutians and extensive ice-free areas in Kam- 
chatka, biota surviving the pleistocene are 
presumed to have spread to adjacent areas as 
the ice withdrew. The most highly plastic 
species were the first to re-establish them- 
selves on such places as the central Aleutians, 
which Hulten thinks were heavily glaciated 
and hence mostly devoid of plant life during 
the glacial periods. Through such differential 
495 
migrations of plants from both ends of the 
Aleutian Arc due to unequal spreading of 
plastic and rigid species, the present Aleutian 
flora came into being. Thus, the characteristic 
middle-Aleutian gap which occurs in the dis- 
tribution of many species (Hulten, 1937^.' 
348-381) is the result of the inability of rigid 
types to invade the islands as quickly as 
plastic ones. 
Hulten’s methods of arranging the equi- 
formal progressive areas are open to certain 
criticisms (Raup, 1947: 231): (1) additions 
to his lists are undoubtedly forthcoming and 
may alter some equiformal areas; (2) many 
of the gaps in plant distribution can be shown 
to be gaps in exploration; and (3) Hulten’s 
sorting of species to illustrate equiformal 
areas may represent a faulty technique. How- 
ever, such criticism is comparatively minor, 
and none of the recent work has seriously 
altered Hulten’s general hypothesis. 
An example of the second criticism of Hul- 
ten’s work is found in our own studies. We 
are now able to fill in many gaps of distribu- 
tion of Aleutian plants. Eriophorum russeolum, 
for example, was supposed by Hulten not to 
occur between Umnak in the eastern Aleu- 
tians and Kamchatka, but we have since 
found it on Atka, Adak, Kiska, and Attu 
(Fig. 1). Likewise, Prunella vulgaris var. aleu- 
tica occurs on several islands of the middle 
Aleutians, where it was supposedly absent. 
Thus, of 105 species which theoretically have 
had distributional gaps in the middle Aleu- 
tians, 24 have been found on islands within 
the gaps. It seems likely that more will turn 
up as further exploration covers additional 
ground. 
Such new and important range extensions 
indicate that much additional field explora- 
tion is needed. Hulten’s supposition that the 
middle islands were heavily glaciated during 
the Pleistocene is not entirely correct, because 
subsequent work has indicated that a number 
of ice-free islands probably existed (U.S.G.S., 
1947): for example, west of Adak, the flat, 
platform islands of Ogliugla, Ilak, Kavalga, 
