OF LIFE IN GENERAL. 313 



ences appeared in the first year. Similarly-, seeds 

 from Germany exhibited analogous differences." * 

 Dr. Schubeler also observed an increased greenness of 

 the foliage. 



The Arctic climate, though similar in many respects 

 to the Alpine, yet differs considerably in others. By 

 comparing plants from the Islands of Spitzbergen and 

 Jan Mayen, with specimens of the same species col- 

 lected in the Alps and the Pyrenees, Bonnier f has 

 shown that there are modifications of structure cor- 

 responding to these differences of environment. The 

 Arctic plants have more rounded cells and more con- 

 siderable intercellular spaces in their tissues, whilst the 

 cuticle is diminished in thickness, and the vessels are 

 diminished in number and in calibre. These changes 

 towards an incipiently aquatic type are probably due to 

 the greater humidity of the air. The fleshiness of the 

 leaves Bonnier attributes to the continuous solar illumi- 

 nation, though it may perhaps be due to the neighbour- 

 hood of the plants to the sea. 



The effect of cultivation on the variation of plants is 

 well known to be in many cases exceedingly great; but 

 in hardly any of the recorded cases is any mention made 

 of the extent to which artificial selection was practiced. 

 One cannot tell, therefore, how much ought to be at- 

 tributed to the direct action of the environment, and 

 how much to selection. The following instance, how- 

 ever, seems to be the direct result of cultivation. 

 It concerns the spiderwort, Tradescantia virginica. 



* Quoted from Henslow's "Origin of Plant Structures," p. 

 118. 

 fRev. Gen. Bot., vi. p. 505, 1894. 



