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 diree- 

 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-conditions 

 are on record for the most various characters 

 and qualities. 



The second point, however, is as difficult to 

 prove as the first is of easy treatment. If aft- 

 er hundreds and thousands of years of exposed 

 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 



