ORIGIN AND SIGNIFICANCE OF SPINES 41 



in other primary causes of spine genesis, there may also come 

 secondary influences of protection, offence, etc., controlled by 

 natural selection. 



It will be convenient to consider spine production from lack 

 of growth force under three heads : 



D. Deficiency of Growth Force. 



D 1. Intrinsic suppression of structures and functions. 



D 2. Disuse. 



D 3. Secondarily for protection, etc. 



Summary of Causes of Spine G-enesis. 



Before taking up in more detail the various causes of spine 

 development, and illustrating them by means of examples 

 drawn from a number of classes of organisms, it is well to 

 restate the factors which are believed to induce spine growth. 

 This is especially desirable from the fact that, through the 

 operation of unlike forces, similar conditions may produce 

 the same morphological results, as in the differentiation of 

 ornamental lamellse and ridges, which, either from external 

 stimuli or dispersion of growth force, may develop into 

 spines. In such cases it is difficult or impossible to distin- 

 guish the primary force, and the only satisfactory method is 

 to discuss the subject under one head. 



By carrying out this plan, and indicating the instances 

 where the causes may replace or overlap each other, it may 

 be shown how spines have originated, as follows : 



I. In response to stimuli from environment acting on most 

 exposed parts. (Ai.) 



II. As extreme results of progressive differentiation of 

 previous structures. (A 2 , B 3 .) 



III. Secondarily, as a means of protection and offence. 

 (A 3 , B 4 , C 4 , D 3 .) 



IV. Secondarily from sexual selection. (A 4 , B 4 , C 4 , D 3 .) 



V. Secondarily from mimetic influences. (A 5 , B 4 , C 4 , D 3 .) 



