THE STRUGGLE FOR EXISTENCE 147 
in the case of what are called " weeds. " Every one is fa- 
miliar with the fact that if cultivated ground is neglected 
these undesirable plants will invade it vigorously and seri- 
ously affect the development of plants under cultivation. 
107. Adaptation. When the changes mentioned above 
occur in the environment of plants to such an extent as 
to make the conditions for living very unfavorable, one 
of three things is likely to occur, adaptation, migration, 
or destruction. 
The change in conditions may come slowly enough, and 
certain plants may be able to endure it long enough to 
adjust themselves to it. Such an adjustment may involve 
changes in structure, and probably no plants are plastic 
enough to adjust themselves to extreme and sudden changes 
which are to be comparatively permanent. There are 
plants, such as the common cress, which may be called 
amphibious, which can live in the water or out of it without 
change of structure, but this is endurance rather than 
adaptation. Many plants, however, can pass slowly into 
different conditions, such as drier soil, denser shade, etc., 
and corresponding changes in their structure may be noted. 
Very often, however, such plants are given no opportunity 
to adjust themselves to the new conditions, as the area is 
apt to be invaded by plants already better adapted. While 
adaptation may be regarded as a real result of changed con- 
ditions, it would seem to be by no means the common one. 
108. Migration. This is a very common result of 
changed conditions. Plants migrate as truly as animals, 
though, of course, their migration is from generation to 
generation. It is evident, however, that migration cannot 
be universal, for barriers of various kinds may forbid it. 
In general, these barriers represent unfavorable conditions 
for living. If a plant area with good soil is surrounded by 
a sterile area, the latter would form an efficient barrier to 
migration from the former. Plants of the lowlands could 
not cross mountains to escape from unfavorable conditions. 
