PEOTECTIVE ADAPTATIONS IX PLANTS 131 



of the species. Even the protective bitter substances (tannin-com- 

 pounds) are not essential to the constitution of the genus Lactuca . 

 their formation may be discontinued without the plant being other- 

 wise affected. And in this case it is not a question of the withdrawal 

 of something which has to be taken in from outside, it is the non- 

 development of what is purely a product of the internal metabolism. 



The adaptations of plants against snails are instructive in another 

 way, namely, in their extraordinary diversity. Here again we see 

 how great is the plasticity of organic forms, and how precisely, 

 though in many very different ways, they adapt themselves to the 

 conditions of their life, in this case the weaknesses of their greedy 

 enemies, and all to attain the same end, the security of their existence 

 as a species. We see at the same time that innumerable minute 

 details in the structure and character of a species, which may appear 

 unimportant, may yet have their definite uses — hairs, bristles, and 

 raphides, as well as bitter substances, ethereal oils, acids, and tannin- 

 compounds. But we must, of course, have minute and exhaustive 

 investigations, like those of Stahl, in regard to the biological relations 

 of these peculiarities before their utility can become clear to us. 



