172 BOTANICAL GAZETTE [MARCH 
plant and animal agencies. These are found to have an influence 
that is more diversified than is the case with the physiographic ° 
agencies; furthermore, their influence can be more exactly studied, 
since they are somewhat readily amenable to experimental control, 
but particularly because they operate with sufficient rapidity to 
be investigated with some exactness within the range of an ordi- 
nary lifetime. If, in their operation, regional agencies are matters 
of eons, and topographic agencies matters of centuries, biotic 
agencies may be expressed in terms of decades. 
It has been seen that changes of climate or of topography gener- 
ally institute vegetative changes; indeed this would have been 
predicted to be the case, even without examination. But at first 
thought it seems somewhat striking that far-reaching vegetative 
changes take place without any obvious climatic change and with- 
out any marked activity on the part of the ordinary erosive factors. 
Indeed, it is probably true that the character of the present vege- 
tative covering of the earth is due far more to the influence of these 
relatively silent and subtle factors than to the more obvious factors 
previously considered. So rapid is the action of the biotic factors 
that not only the climate, but even the topography may be regarded 
as static over large areas for a considerable length of time. It has 
been said that many of our Pleistocene deposits exhibit almost the 
identical form which characterized them at the time of their deposi- 
tion; in other words, the influence of thousands of years of weather- 
ing has been insufficient to cause them to lose their original 
appearance. These thousands of years would have sufficed for 
dozens and perhaps for hundreds of biotic vegetative cycles. Many 
a sand dune on the shores of Lake Michigan is clothed with the cul- 
minating mesophytic forest of the eastern United States, and yet 
the sand dunes are products of the present epoch; furthermore, sand 
is regarded generally as a poor type of soil in which to observe rapid 
succession. If a clay upland were denuded of its forest and its 
humus, it is believed that only a few centuries would suffice for 
the mesophytic forest to return. 
From the standpoint of dynamic plant geography our land areas 
are divided into two well marked categories: on the one hand 1s 
the erosion topography that is characteristic of the eroding 4” 
