184 Chapters in Modern Botany CHAP. 



are highly specialised leaves. Again, we saw that the 

 Clematis and many other plants are leaf-climbers, and that 

 tendrils are in many cases modified leaves or parts of 

 leaves. 



In not a few plants the leaves die away, in whole or in 

 part, often becoming reduced into mere spines. The Dar- 

 winian explanation of course is that these spines have 

 been produced by natural selection in consequence of 

 preserving the plants from being eaten up by mammals. 

 The Lamarckian explanation, on the other hand, is that 

 spiny plants are the product of the conditions of drought in 

 which the majority at least occur ; while the writer has else- 

 where 1 offered an interpretation complementary to this of 

 thorny species as being of ebbing vitality as compared with 

 their thornless congeners. The reader will find a vigorous 

 criticism of this view from the Darwinian standpoint in 

 Mr. Wallace's Darwinism, and (if he thereafter retains 

 sufficient interest in these Lamarckian and neo-Lamarckian 

 views) a briefly summarised reply in the British Association 

 Report for 1890. The controversy would lead us beyond 

 the limits of this little volume ; suffice it here again to say 

 that in thinking of this and many similar problems in 

 evolution, the student should be careful to distinguish 

 between the primary factors, which really originate the 

 peculiarity in question, and the secondary factors, which 

 determine whether the peculiarities shall or shall not 

 persist. Natural Selection of course is the general name 

 for all these secondary agencies. 



Here again is a matter for direct experiment, which has 

 indeed begun. 2 Thus Lothelier has shown with barberry 

 and hawthorn (i) that the promotion of transpiration in- 

 creases the development of thorns which were absent when 



1 VARIATION, Ency. Brit., and Life-Lore, 1889. 



2 Comptes Rendus, cxii., 1891. 



