52 STUDIES IN EVOLUTION 



The raised lines or ridges on the legs and carapaces of 

 Crustacea are frequently spiniferous; as Grelasimus princeps, 

 Greearcinus ruricola, etc. The radii on the shells of barnacles 

 are sometimes differentiated into spines ; as in Balanus tintin- 

 nabulum var. spinosus.^^ 



In the higher animals the differentiation of ornamental 

 features into spines is not common, especially as most of the 

 forms are devoid of hard external parts. Among the fishes 

 and reptiles certain lines and ridges on the head and body 

 are often spiniferous, while in others the scales have spi- 

 niferous ribs. 



III. 8eco7idarily as a means of protection and offence. 

 (A3, B4.) 



After spines have originated through the stimuli from the 

 environment acting on the most exposed parts, or by growth 

 force, or by progressive differentiation of previous structures, 

 they may often acquire added qualities, one of which is to 

 protect an organism from the attacks of many of its enemies. 



Morris ^^ shows that defence in animals is either mechanical 

 or motor, while in the higher plants it is purely mechanical. 

 The spine clearly belongs to the mechanical mode of defence, 

 and in many animals may be efficient without motion. If 

 motion is added, it then may serve not only for protection 

 but for offence as well. Natural selection evidently could 

 not originate a spine, but after one has appeared from any of 

 the causes mentioned in the preceding paragraph, this agency 

 could tend to preserve and allow the spine to develop along 

 certain lines. The restrictions as a defensive structure would 

 be those of efficiency, and therefore all the monstrous growths, 

 vagaries, and ornamental spine features would arise indepen- 

 dently of the action of protective selection, and would be 

 accounted for by the operation of the forces of the environ- 

 ment, growth, and sexual selection. In this way the simple 

 antlers of the Tertiary Deer may be imagined to have reached 

 the highest degree of efficiency as weai:)ons, by ordinary nat- 

 ural selection (figure 41). In most cases the subsequent 



