THE STKL'GGLE FOR EXISTENCE 147 



in the case of what are caUed "weeds." Ever}' one is fa- 

 miliar witli the fact that if cultivated ground is neirlected 

 these undesiralile plants will invade it vigorously and seri- 

 ously allect the development of plants under cidtivation. 



lUT. Adaptation. — \\ hen the changes mentioned al)ovo 

 occur in the environuR'nt of ]ilants to such au extent as 

 to make the conditions for living very unfavorable, one 

 of tliree things is likely to occur, adaptation, nugration, 

 or destruction. 



The change in conditions may come slowly enough, and 

 certain plants may tie ahle to endure it long enough to 

 adjust themselves to it. Such an adjustment may involve 

 changes in structure, and ]iroljably n(j plants are plastic 

 enough to adjust themselves to extreme and sudden changes 

 which are to be comparatively permanent. There are 

 plants, su(/li as the common cress, which nuiy be called 

 amphil)ious, which can live in the water or out of it without 

 change of structure, but this is endurance rather than 

 achiptation. Manv plants, however, can pass slowl)' into 

 different conditions, such as drier soil, denser shade, etc., 

 and corresponding changes in their structure may be noted. 

 Yerv often, however, such plants are given no op])ortunity 

 to adjust themselves to the new conditions, as the area is 

 apt to Ije invaded liy 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. 



lOS. Migration. — 'J'his is a very common result of 

 changed conditions. Plants migrate as truly as animals, 

 though, of course, their nugration 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 efHcient barrier to 

 migration from the former. Plants of the lowlands could 

 not cross mountains to escape from nnfavorable conditions. 



