148 PLANT STUDIES 



'J'o make migration jxissiljlc, tliorofore, it is necessary for 

 the couditions to Ijo favoraltle for tlie migrating plants in 

 some direction. In the case of Imlrnslies, cat-tail Hags, 

 etc., growing in tlie slioal watci- of a lake margin, tlie 

 Lnilding np of soil al)out tliem results in unfavorable con- 

 ditions. As a consciiuencc, they migrate fiii'thcr into tlic 

 lake. If the lake liappens to be a small one, the filling nj] 

 })rocess may finally ol)literate it, ami a. time will come when 

 such forms as bulrushes and Hags will find it impossiljle to 

 migrate. 



lu glacial times very many arctic plants migrated south- 

 ward, especially along tlie mountain systems, and many 

 al})iue plants moved to lower ground. When warmer con- 

 ditions returned, many ])lants that had been diiven south 

 returned towards the north, and the atretic and alpine jilants 

 retreated to the noi'tli aiul up tlie mountains. The history 

 of plants is full of migi-a.tions, compelled by changed con- 

 ditions and jiermittcd in various directions, it must lie 

 remember'ed, also, tliat migrations often result in changes 

 of structure. 



l()!i. Destruction. — Prot)ably this is by far tlie most com- 

 mon result, of greatly idianged couditions. Even if plants 

 adapt theiirselves to changed conditions, or migrate, their 

 structure may be so changed tha.t they will seem like quite 

 different plants. In this way old forms gradually disappear 

 and new ones take their jilaccs. 



