TRANSACTIONS OF SECTION K. 669 
rarely stable, and that in general ‘ edaphic formations’ (as Schimper termed 
them) tend to develop into the ‘climatic formation’ of the particular 
region studied. For example, east of Manitoba all other associations 
tend to develop into a mesophytic conifer forest, whereas to the westward 
all other associations tend to develop into prairie. The author feels, in 
view of the increasing objection to the use of the terms ‘edaphic’ and 
‘climatic,’ that it may be well to use such terms as ‘ proximate’ and 
‘ultimate.’ Besides, these terms explicitly imply evolution, which is 
not the case with the others. Stages between proximate and ultimate may 
be denominated approximate, mediate, and penultimate. 
Having satisfied ourselves of the reality of succession, the next step 
is the investigation of the underlying causes. It is probable that succes- 
sion does not take place where there is no essential change in external 
conditions. Such stability of conditions, however, is extremely rare except 
in those situations where the ultimate formation has come to full develop- 
ment, or where the proximate and ultimate formations are the same, as 
in deserts. The simplest as well as the most important changes that 
bring about succession are those which occur where the topography is stable 
and are associated more or less directly with the plants themselves. 
Wherever plants grow in any abundance there is an accumulation of 
humus, relatively great in amount in cold, moist climates and small in 
amount .in hot, dry climates. Humus accumulation of necessity induces 
profound changes in the soil. Foremost among these are changes in water 
content. On uplands, especially those composed of rock or sand, humus 
accumulation means an increase of soil moisture, and thus makes possible 
the development of a vegetation less and less xerophytic, while in depressions 
such accumulation means decreasing soil moisture and the consequent 
elimination of hydrophytes. Humus is a further factor in succession in 
that it furnishes a fit habitat for saprophytes, which make symbiosis 
possible; it is likely that the late appearance of the beech in the succes- 
sion series is due in some part to its mycotrophic relations. Recently 
another possibility has been suggested which may be considered in connec- 
tion with humus. It appears likely, from experiments on wheat, that 
root excretions are detrimental to further root activity, especially among 
plants of the same species. If this is generally true, it is clear that succes- 
sion is facilitated or even necessitated. Again the increase of humus alters 
the food conditions in the soil, more and more favouring such plants as 
require the presence of special organic ingredients. An increase of humus 
also involves changes in the air content and temperature of the soil that 
may be of some moment, especially in bogs or moors, plants of mesophytic 
structure being able to replace the so-called bog-xerophytes. 
Searcely second to humus is the influence of increasing or decreasing 
shade. Proximate upland associations are exposed to maximum illumina- 
tion, while each succeeding stage is characterised by an increase of shade, 
which favours those plants requiring shade for germination and opposes 
those requiring light. The ultimate formation of any upland will be 
composed of those plants that can germinate in the densest shade that 
may there exist. An increase of shade is also accompanied by increasing 
humidity and soil moisture, thus working in harmony with the humus 
factors. 
Plant invasions influence succession inasmuch as new elements are 
introduced. Similarly the influence of man on succession is very con- 
siderable, partly by reason of the plants which are enabled to invade 
new regions through his agency, but more by reason of his destructive 
activities. In nearly every instance man retards the accumulation of 
humus and increase of shade, thus tending to keep upland associations 
more xerophytic than would otherwise be the case. 
Topographic changes are of profound importance in succession. Winds, 
