148 PLANT STUDIES 
To make migration possible, therefore, it is necessary for 
the conditions to be favorable for the migrating plants in 
some direction. In the case of bulrushes, cat-tail flags, 
etc., growing in the shoal water of a lake margin, the 
building up of soil about them results in unfavorable con- 
ditions. As a consequence, they migrate further into the 
lake. If the lake happens to be a small one, the filling up 
process may finally obliterate it, and a time will come when 
such forms as bulrushes and flags will find it impossible to 
migrate. 
In glacial times very many arctic plants migrated south- 
ward, especially along the mountain systems, and many 
alpine plants moved to lower ground. When warmer con- 
ditions returned, many plants that had been driven south 
returned towards the north, and the arctic and alpine plants 
retreated to the north and up the mountains. The history 
of plants is full of migrations, compelled by changed con- 
ditions and permitted in various directions. It must be 
remembered, also, that migrations often result in changes 
of structure. 
109. Destruction, Probably this is by far the most com- 
mon result of greatly changed conditions. Even if plants 
adapt themselves to changed conditions, or migrate, their 
structure may be so changed that they will seem like quite 
different plants. In this way old forms gradually disappear 
and new ones take their places. 
