1922] Peattie,—Coastal Plain Element in Flora of Great Lakes 69 
promontories. This stability is so favorable to the advance of a vege- 
tation more vascular than that which ordinarily characterizes the 
coastal plain, that thickets and forests are slowly taking possession 
on those shores and the coastal plain types are being crowded out. 
It is noticeable that those species which show the greatest discon- 
tinuity of range, as exemplified by Eleocharis melanocarpa, are in 
most cases inhabitants of the more transitory of the littoral physio- 
graphic features—the ponds, pond-margins, and lagoons; while it is 
those plants of the second type, plants of dunes and strands such as 
Euphorbia polygonifolia, which have tended to persist along the Great 
Lakes. The discontinuity of the first type would point to a dying- 
out, and the persistence of the type of dunes and strands merely means 
that while there are any Great Lakes, there must always be a strand, 
and also there will usually be dunes. 
In general, the coastal plain types persist only where those younger 
and less-permanent features of the shore erosion cycle are still to be 
found, and they themselves represent the younger stages of plant 
succession. Such is the case at Presque Isle, where the whole island 
has been shown by direct observations over the course of a century 
to be moving eastward, the west shores being washed away by a 
prevailing current, and the east being built out by spits. And the 
Indiana shores of Lake Michigan represent a long series in the cycles 
of shore erosion and more particularly of lake recession. One may 
observe there all stages from new barrier beaches and lagoons to 
shores which have been dry of water for thousands of years. 
The coastal plain element follows closely the shores of the lakes 
as they recede, and the field and forest types of the adjacent country 
press hard upon it; indeed the coastal plain plants prepare the ground 
for their successors. By the larger measures of time, the coastal 
plaia element in this region is rather ephemeral, though if the balance 
of nature is not disturbed by man, it will probably never be entirely 
eliminated. This pioneering quality of the coastal plain type is, really 
a further proof of the early migration around the glacial lakes. These 
plants probably then were, for the same reasons that they now are, 
biologically well adapted to pressing forward quickly on the unstable 
margins of these lakes, while the forest types would naturally return 
more slowly. 
