448 Ever-sporting Varieties 



such, a way .as to make them better adapted to 

 life, under prevailing circumstances. Nageli 

 gave to this conception the name " Theory 

 of direct causation" (Theorie der direc- 

 ten Bewirkung), and it has received the ap- 

 proval of Von Wettstein, Strasburger and other 

 German investigators. According to this con- 

 ception a plant, when migrating from lowlands 

 into the mountains would slowly be changed 

 and gradually assume alpine habits. Once ac- 

 quired this habit would become fixed and attain 

 the rank of specific characters. In testing this 

 theory by field-observations and culture-exper- 

 iments, the defenders of the Nagelian principle 

 could easily produce evidence upon the first 

 point. The change of lowland-plants into al- 

 pine varieties can be brought about in numer- 

 ous cases, and corresponding changes under 

 the influence of soil, or climate, or life-condi- 

 tions are on record for the most various charac- 

 ters and qualities. 



The second point, however, is as difiScult to 

 prove as the first is of easy treatment. If after 

 hundreds and thousands of years of exposure to 

 alpine or other extreme conditions a fixed 

 change is proved to have taken place, the ques- 

 tion remains unanswered, whether the change 

 has been a gradual or a sudden one. Darwin 

 pointed out that long periods of life afford a 



