296 BOTANICAL GAZETTE [OCTOBER 
determinable influence upon the vegetation. An animal impor- 
tant in the economy of soils, namely the common earthworm, is 
said by the residents not to occur upon the marshes. I have 
seen what appeared to be their castings, but have been told by 
an intelligent observing farmer that these are in reality made by 
some burrowing insect. 
Geography of the basin—To the ecological factors already 
considered there must be added another of a different sort, 
namely, the geography of the basin itself. We have already con- 
sidered one phase of this subject, for upon the latitude, elevation, 
proximity to the sea, to cold or warm ocean or air currents, 
depend some of the ecological factors previously discussed. 
. But in addition there are two other important phases of the 
subject. First, there is the geographical position of the region 
relative to the great floristic divisions of the earth’s surface, 
upon which depends its flora (as distinct from its vegetation, 
which is determined by the preceding), and the flora determines 
the materials upon which the particular ecological factors of the 
region are to work. In this case we are dealing with a portion 
of the region covered by the temperate North American flora, 
with all the peculiarities of species, genera, and families thereto 
belonging. Second, there is the degree of isolation of the 
basin from the neighboring regions. Isolation may be brought 
about principally by the presence of natural barriers, mountain 
ranges, wide arms of the sea or desert, or even to some extent 
by great size. Isolation is ecologically important, for upon it 
depends the possibility of the rapid development of indigenous 
and exceptionally adapted forms. It produces this result both by 
preventing the dilution of new adaptive characters through 
crossing with immigrants from without, and also by preserving 
undiluted those characters which may be developed independ- 
ently of adaptation. Regarding now the marsh country from 
this point of view, we see at once that it is entirely without 
natural barriers of any kind, and lies, open in every direction to 
immigration from sea-shore, field, and forest; while it is so small 
in extent (nowhere exceeding four or five miles in diameter.), 
that the natural modes of locomotion of most of the plants 
