42 GrRIGGS: OBSERVATIONS ON THE BEHAVIOR 
covered by a mixed forest of scrub and pitch pine, black oak and 
blueberries. Through the whole region much of the land has 
been abandoned and not more than half of the houses are now 
occupied. The crevices in exposed rocks are the poorest of all the 
habitats in the region and it can be shown that the majority of 
the chasmophytes are plants which have been crowded out of the 
more favorable habitats which they would quickly repopulate, as 
the Avalia has done, were their more successful neighbors 
removed. This very plant indeed, when the forest is removed, 
gives abundant evidence of the kind of habitat it prefers. It is 
really a sun-loving plant and grows only where sunlight is abundant. 
In the primeval forest the upper faces of the cliffs were the only 
situations in which a plentitude of light was available to a plant 
of such humble stature, so here it developed wherever together with 
sunlight sufficient water was present. 
Even so few examples as these are sufficient to destroy the 
utility of the theory that at the edges of their ranges, species are 
confined to the most favorable habitats. Where it describes 
conditions previously known to obtain, well and good. But it 
cannot be generalized nor can it serve as a guide to the optimum 
habitats of plants whose preferences in this matter are unknown. 
Like much @ priori reasoning it presents a conclusion which may 
be true rather than one which must be true. Moreover, another 
line of a priori reasoning might in this case lead us to diametrically 
opposite conclusions. In the center of its range a plant, being — 
supposedly favored by all the conditions of its environment, is 
able to compete with other plants at an advantage and maintain 
a place for itself in various habitats. On its areal limits on the 
other hand environmental conditions are supposedly less favorable 
to this species than to others with which it will therefore be com- 
pelled to compete at a disadvantage. This may be brought 
about either by unfavorable physical conditions such as. soil, 
climate, etc., or by the entry into the struggle for place of species 
absent from the center of the range which have an advantage 
over the given species in rapidity of growth or the like. The 
species in question is therefore driven out of the most favorable 
habitats and must find a place for itself where it can. This is 
exactly the situation of Aralia spinosa which in the dense forest 
