CH. VI] ADAPTATION 57 



of Europe, with no success except in the high mountains, where 

 many herbaceous, but rarely arboreous, things have taken a hold 

 upon ground from which the original plant associations had been 

 removed. In the same way they tried to acclimatise in Europe 

 tropical things like the dahlia or the potato, but even after the 

 lapse of centuries these plants remain "half-hardy". In both 

 these cases the change of conditions was too great to allow of 

 physiological adaptation, which might perhaps have taken place 

 in a gradual acclimatisation over a very long period of time with 

 only very slight alteration in conditions at each step. Or it may 

 have been only that the range of capacity to withstand conditions 

 was not sufficient even after the utmost had been done in accli- 

 matisation. Time and gradual progression are the most essential 

 things in acclimatisation. 



A very great difficulty in the path of acceptance of natural 

 selection as a cause for gradual adaptation is the fact that so 

 many of what look like real morphological adaptations require so 

 much correlation. Climbing plants come into this group, though 

 they are obviously well suited to climbing. The habit cannot be 

 difficult to acquire, for there are so many cases of the closest 

 relatives, one climbing, one erect. A climber also needs a support, 

 which is usually an erect plant, so that erect plants must have 

 been the earlier. But one cannot imagine natural selection 

 picking out the beginnings of weak and flexible stems, whether 

 by gradual change or by small mutations. And when at last they 

 were formed, as obviously there would be no value in developing 

 tendrils or other means of climbing until the stems were weak, 

 they would collapse into the darker lower levels of vegetation, 

 and would have to undergo physiological adaptation to living in 

 greater darkness. Then they would have to learn to form climbing 

 organs, and finally, learning to climb, they would once more have 

 to adapt themselves to life in greater light. And what use would 

 the beginnings of tendrils or other climbing organs be? And why, 

 after having learnt to live in greater darkness, should the plant 

 want to grow up into the light once more? Yet it would be dragged 

 up by the tendrils, and would probably suff'er from the excess of 

 light. There is too much, and too complicated internal adaptation 

 required, to say nothing of the external. One must look with 

 great suspicion upon such an easy interpretation of such struc- 

 tural features as climbing stems as being simply adaptations. If 

 they were gradually formed, the work was too complicated for 

 natural selection to perform. 



