Double Adaptations 451 



stems, a dense foliage, a strongly cuticnlarized 

 epidermis, few and narrow air-cavities in the 

 tissues and all the long range of characteris- 

 tics of typical desert-plants are not a simple 

 result of the influence of climate and soil. 

 There is no direct influence in this sense. 



The second point, in which Nageli's idea is 

 broken down by Holtermann's observations, re- 

 sults from the behavior of the plants of the 

 Kaits desert when grown or sown on garden- 

 soil. When treated in this way they at once 

 lose the only peculiarity which might be con- 

 sidered as a consequence of the desert-life of 

 their ancestors, their dwarf stature. They be- 

 have exactly like the alpine plants in Bonnier's 

 experiments, and with even more striking dif- 

 ferences. In the desert they attain a height 

 of a few centimeters, but in the garden they 

 attain half a meter and more in height. Noth- 

 ing in the way of stability has resulted from the 

 action of the dry soil, not even in such a minor 

 point as the absolute height of the stems. 



From these facts and discussions we may 

 conclude that double adaptation is not induced 

 by external influences, that is to say, not by 

 those which might be of use to the plant. 

 It may arise by some unknown cause, or may 

 not be incited at all. In the first case the plant 

 becomes capable of living under the alternat- 



