46 GRIGGS: OBSERVATIONS ON THE BEHAVIOR 
in the richest of deep alluvial bottomland soils as in the canyon 
of Queer Creek in the southern section of the present area. In 
the northern section of the area, however, where the valleys are 
somewhat older in a physiographic sense, the hemlock is restricted 
to the rocks around the edges of the ravines while the bottom is 
occupied by a mixed Liriodendron forest. A study of the plant 
societies here has convinced me, however, that it once occupied 
the bottoms as it still does in the more inaccessible valleys further 
south, but that it has been driven out of the more favorable 
habitats by the hardwoods. It seems probable that it once 
ranged continously over all of Ohio and Indiana but that it has 
been displaced except in a few outlying rocky stations where it 
has been able to maintain itself till the present time. Study of 
the reproduction of the hemlock forest in the present area would 
indicate moreover that under present conditions, as modified 
by man, it will soon disappear from these stations as it has from 
the surrounding territory. On account of the superficial root- 
system, seedlings are unable to get a start except in very humid 
conditions such as obtained in the virgin forest. Wherever the 
humidity has been lowered by clearing, the hemlock seedlings 
are quickly dried out and killed both on rocks and in the bottom, 
while the hardwood trees which spring up beside them thrive 
where the soil is sufficiently deep. 
The reasons for the increased abundance of such a plant as 
Aralia spinosa are of course not natural but artificial, introduced 
by the advent of man. On first thought one might be inclined to 
rule out such cases, but clearing and cultivation of the land make 
little if any change in climatic conditions. Their effect is in chang- 
ing the conditions of competition between plant species. How 
profound this influence has been is forcibly brought to our attention 
every day by our pestiferous weeds, many of which are native 
species originally present in the forests but in insignificant numbers- 
All of these are species with some effective means of seed dispersal 
and their conquest of the country is now so complete that it is 
impossible to imagine their aboriginal condition. It is only the 
slow-moving species whose progress can be now studied. : 
That plant competition often plays a larger part than climatic 
influences is also indicated by the fact that most plants thrive 
ase ii 
