3 2 3 
A. G. Tansley. 
insight and the generous spirit to subordinate motives of immediate 
material profit to higher and deeper considerations. Comparatively 
few may be found as yet to pay this honour, but those few will 
become many as time goes on, and the general level of appreciation 
of the permanent and underlying factors of human life in its 
widest sense is gradually, as it will inevitably, be raised to a higher 
and higher level. 
Effect of Moving Sand-Dunes on Forest Vegetation. 
All the stages of the sand-dune succession subsequent to the 
cottonwood stage are liable to be overwhelmed by moving masses 
of sand, 100 feet or so high, and the sequence of events that occurs 
when the later forest stages are thus invaded is most interesting. 
If the advancing face is moving at a comparatively slow rate, many of 
the trees and shrubs of the forestare not killed by the sand, but are able 
to grow up pari passu with the advance, and keep leafy shoots always 
above the surface, e.g ., species of Rhus, Tilia americana, Prunus 
serotina, Vitis vulpina, etc. This power depends upon the capacity 
to strike adventitious roots into the layer of damp sand which is 
found a few inches from the surface throughout the year. It is 
this capacity which enables the cottonwood ( Populus deltoides ) 
to dominate the less rapidly moving dunes. The seeds of this tree 
can only germinate in the very damp soil of the shallow depressions 
near the beach where the bottom water nearly reaches the surface, 
or in places with a similar soil-water content on the edges of 
pools. The seedlings can, however, hold their own by the rapid 
upgrowth of their shoots through successive layers of invading 
sand, into which, as soon as it becomes moist, adventitious roots 
are sent (provided, of course, that the onset is not too rapid); and 
thus many of the lower but still considerable dunes near the sea 
are loosely covered with flourishing cottonwood trees which started 
life at a level sometimes scores of feet below the existing surface. 
Where wind erosion has subsequently destroyed part of such a 
dune the eroded face shows the old stems and adventitious roots of 
the cottonwood at all levels. 
When a large moving dune invades a forest belonging to one 
of the later stages of the dune succession, some of the trees, e.g., 
Piuus Banksiana and P. Strobus are killed very soon, and neither 
the oaks nor the sugar-maple ( Acer saccliaruni) can survive very 
long; but the trees and shrubs mentioned above maintain them¬ 
selves by keeping pace with the rising sand. Accordingly we have 
