118 Harper: Trip ON WARRIOR AND TOMBIGBEE RIVERS 
tury or two ago. Those which were seen more than twice have 
already been listed above with the native species, without any 
attempt to separate them. These will be mentioned again below, 
together with the less frequent weeds. 
For the occurrence of so many weeds (about 40 species were 
noted on the trip) in such out-of-the-way places the following 
explanation is suggested: The edaphic conditions on the banks 
are very diverse, each subdivision of the Cretaceous and Tertiary, 
as well as the Quaternary second bottoms, furnishing one or more 
different types of rock or soil, some of which extend only a few 
feet vertically and a few hundred yards horizontally. In addition 
to this, the different bluffs face all points of the compass, so that 
there is a change of environment with every bend in the river. 
Now if every different formation (and some of them are very 
different from anything exposed on level ground away from the 
rivers) had one or more peculiar species adapted to it by processes 
of evolution, every such river would be bordered by many endemic 
and very local species. But species (fortunately, one might say) 
do not seem to be produced quite so freely, and few native plants 
have been able to grow on the faces of the bluffs at all. The 
existence of a species confined to a particular kind of river-bluff 
would be rather precarious, anyway, for the faces of bluffs 
frequently slough off into the river, destroying all the vegetation 
on the area affected. 
But most weeds are already adapted to diverse soil conditions, 
and a river is an excellent highway for plants to travel, so that 
those species which gain access to the river from the fields and 
settlements along it quickly take possession of the unoccupied 
bluffs where native plants have been unable to establish themselves. 
It should not be inferred from this statement however that the 
bluffs are now completely covered with weeds. A large part of 
their area is usually too hard or too steep, or crumbling away 
too rapidly, to afford a foothold for any kind of vegetation, and 
consequently the bluff weeds are confined chiefly to crevices or to 
gentle slopes near the base. 
Not all of the river-bank weeds are exotics. There seem to 
be all gradations (if such a thing were possible) between species 
known to have been introduced from distant parts of the earth and 
