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. j\lany 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. ^\'hile 

 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 eflBcient barrier to 

 migration from the former. Plants of the lowlands could 

 not cross mountains to escape from unfavorable conditions. 



