68 Rhodora [APRIL 
As for the possibility of migration before that time, there is some 
evidence that migration also took place in the Lakes Chicago and 
Warren stage, shown in Figure 3. The reason for thus thinking 
is that so many coastal plain species are found in the present valley 
of the Grand River in Michigan which, during the stage just referred 
to (but not later) formed the only connective between Lake Chicago 
and Lake Warren. According to Leverett and Taylor, this connective 
ceased to function after this stage. Hence any plants which now 
persist along the Grand River Valley probably owe their presence 
to migration during the period earlier than the Algonquin stage, 
when the Grand River connective was functioning. 
On the other hand, it is not perfectly certain that this outlet 
did not function at later periods of time, and it must be admitted 
that, with the glacier very close at hand, the period of the Grand 
River connection was certainly the earliest possible opportunity for 
the migration and ‘s, perhaps, too early to conform with most of the 
facts as we know them. 
Certainly the migration did not occur much later than the Algon- 
quin stage, that is, it is not now going on. This we know because 
the number of stations for localized coastal plain species has not 
materially increased during the period of observation on the region. 
And we know it from the fact that the conditions which favored 
the spread of coastal plain plants—conditions of which we shall speak 
later—soon ceased to exist in any general way after the close of the 
Algonquin. 
PLACE or Coastat PLAIN PLANTS IN EROSION CYCLES AND 
PLANT SUCCESSION. 
It is a remarkable fact that there are many more coastal plain 
species on Lake Michigan, nearly one thousand miles from the At- 
lantic, than on Lake Erie, four hundred miles nearer. This I believe 
to be due to the chance that conditions around the head of Lake 
Michigan are more favorable to the preservation of plants with habits 
like those of the coastal plain. There are rarely found on Lake Erie 
such superlative conditions as exist on Lake Michigan at Pine or 
Dune Park, Indiana. Indeed, the conditions on the Great Lakes as 
a whole have now become fairly stabilized. The supply of water is 
probably in an approximate equilibrium with the loss, so that no 
changes of level, save seasonal ones, are taking place and the currents 
have silted up the bays, while the storms have worn away the weaker 
