804 . Fluctuations 



neighboring contiaent. This local difference is 

 ascribed by Behrens to a more severe selection 

 by the pollinating insects in consequence of their 

 lesser frequency on these very windy isles. 

 Seeds of the pines from the Himalayas yield 

 cold-resisting young plants if gathered from 

 trees in a high altitude, while the seeds of the 

 same species from lower regions yield more 

 sensitive seedlings. Similar instances are af- 

 forded by Rhododendron and other mountain 

 species. According to Cieslar corresponding 

 differences are shown by seeds of firs and 

 larches from alpine and lowland provinces. 



Such changes are directly dependent on ex- 

 ternal influences. This is especially manifest 

 in experiments extenduig the cultures in higher 

 or in more northern regions. The shorter 

 summer is a natural agent of selection; it ex- 

 cludes all individuals which cannot ripen their 

 seeds during so short a period. Only the short- 

 lived ones survive. Schiibeler made very strik- 

 ing experiments with corn and other different 

 cereals, and has succeeded in making their cul- 

 ture possible in regions of Norway where it for- 

 merly failed. In the district of Christiania, corn 

 had within some few years reduced its lifetime 

 from 123 to 90 days, yielding smaller stems and 

 fewer kernels, but still sufficient to make its 

 culture profitable under the existing conditions. 



