12 



STUDIES IN EVOLUTION 



13 



its direct or indirect development cannot be determined. An 

 excellent example of indirect, or regressive, growth of spines 

 is afforded in the common Barberry QBerberis vulgaris}, on 

 the summer shoots of which are shown most of the gradations 

 "between the ordinary leaves, with sharp bristly teeth, and 

 leaves which are reduced to a branching spine or thorn. The 

 fact that the spines of the Barberry produce a leaf-bud in 



their axil also proves them to 

 be leaves "2* (figure 13). 



It should be noted that the 

 process of spine development 

 illustrated in Spondylus (figure 

 14) is directly oj)posed to that 

 of the Barberry. In the former 

 the initial growth is smooth, then 

 faint, concentric, and radiating 

 lines appear, which gradually 

 grow stronger, developing more 

 or less regular inequalities ; and 

 by the excessive growth of these 

 variations spines are formed. In 

 the Barberry there are at first 

 normal leaves, which are followed 

 intUcatls The "direction^ of by others more and more toothed 

 and bristly, until the leaf is rep- 

 resented by a branching spine, 

 while finally spines only are 

 formed. The Spondylus repre- 

 sents a progressive increase in 

 growth to produce the spines, while the Barberry exhibits a 

 progressive decrease of growth, or an "ebbing vitality," as it 

 has been termed by Geddes.^^ 



The spines are the final results of both the direct and 

 indirect modes of production; the direct, through a process 

 of building on new tissue, and the indirect, through a process 

 of dwindling away to all but the axial elements. These 

 differences are graphically expressed in figures 13 and 14. 



Figure 13. — Summer shoot of 

 Barberry ; showing the gradations 

 between leaves and spines. The 

 arrow 

 growth. (After Gray.) 



Figure 14. — Profile of one of 

 the primary rays of Spondylus 

 imperialis ; showing the series of 

 spines. The arrow indicates the 

 direction of growth. 



