1922] THONE—STARVED ROCK 347 
entrance of new species in a given area are operative first on the 
seedling. Liminal conditions act on the infant of the race, prac- 
tically always. The operation of this principle may be seen in the 
cultivation of adult plants outside of their natural habitats. These 
often thrive, but do not produce offspring. By the simple law of 
chance, propagules of all sorts are constantly falling into every area 
within the range of flight or carriage from the parent, but only where 
- conditions are such as to permit their germination and initial 
growth do they become established. Once established they may 
weather an unfavorable season, but they cannot gain a foothold 
at all in a place where the conditions are unfavorable all the time. 
This principle recognized, it becomes at once apparent that any 
measurements undertaken with a view to their bearing upon suc- 
cession should be made with special reference to seedling seasons 
and places. The first worker to use this idea as a definite basis for 
his investigations was FULLER (7), who made a study of the water 
relations of several plant associations on and near the Indiana dunes. 
His results indicated that the water supplying power of the soil was 
fairly uniform, or at least adequate for growth, throughout the 
season at all of his stations. On the other hand, the evaporating 
power of the air varied markedly, showing a pronounced correlation 
with the type of vegetation. The rate of evaporation bore an 
inverse relation to the density of the vegetation, being greatest on 
the cottonwood dune and least in the beech-maple forest. FULLER 
concluded that the differences in evaporation rates were sufficient 
to account for the successional range between the relative xero- 
phytism of the cottonwood dune and the mesophytism of the 
climax forest. 
The present problem 
TERRAIN 
Conditions rivalling those of the dunes in diversity of vegeta- 
tional associations to be found within narrowly restricted limits 
exist in numerous steep-sided river gorges and their associated 
canyons scattered across the whole Mississippi Valley, along the 
edges of the various glacial drift areas, and in the unglaciated areas 
adjoining them. These cliffs and canyons are invariably the 
