62 NATURAL SELECTION FROM 



For example, the character of thorns was said 

 to be selected because their presence was a pro- 

 tection against grazing animals. Now it is 

 known that thorns chiefly prevail among plants 

 in regions peculiarly free from grazing animals ; 

 and that even if the grazing animals are present 

 the thorns do not appear in the early stages of the 

 plant, when they are most needed. Conversely, 

 the plants chiefly attacked by grazing animals are 

 singularly free from thorns. Experimental work 

 has shown that many thorns are a response to 

 poor nutrition, and that they may or may not be- 

 come an estabhshed character. 



The elaborate stinging hairs of the nettle rep- 

 resent a character that according to this view 

 was built up by Natural Selection, with adapta- 

 tion as_..the, principle. o£ selection. Now it is 

 known that the nettle is indifferent to their pres- 

 ence and gets along without them. 



It is a well-known fact that many seeds, espe- 

 cially those of arid regions, develop a testa so 

 hard that it interferes with the breaking through 

 of the embryo. In fact, it is becoming evident 

 that if selection is working in these cases it is 

 working towards " over-adaptation." ^ 



A difficulty is also presented by such structures 

 as the velamen of the aerial orchids, as well as 

 by the water-conducting vessels of the vascular 

 system. In both of these cases the structures do 



^ This situation has been developed by the recent studies of 

 the germination of seeds and spores by Dr. William Crocker of 

 the University of Chicago. 



