C. E. JBeecher — Origin and Significance of Spines. 249 



Art. XXVI. — The Origin and Significance of Spines : A Study 

 in Evolution; by Charles Emerson Beecher. 



[Continued from page 136.] 

 Summary of Causes of Spine Genesis. 



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 lamellae 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 distinguish the pri- 

 mary 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. (A,.) 



II. As extreme results of progressive differentiation of pre- 

 vious structures. (A 2 , B 3 .) 



III. Secondarily, as a means of protection and offense. (A„ 

 B„ C 4 , D,.) 



IY. Secondarily from sexual selection. (A 4 , B 4 , C 41 D 3 .) 

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

 YI. Prolonged development under conditions favorable for 

 multiplication. (B r ) 



VII. By repetition. (B 2 .) 



VIII. Restraint of environment causing suppression of 

 structures. (C,.) 



IX. Mechanical restraint. (C 2 .) 



X. Disuse. (C 3 , D 2 .) 



XI. Intrinsic suppression of structures and functions. (D v ) 



To illustrate the various causes of spine growth, representa- 

 tive examples which are believed to conform to the require- 

 ments will be selected from various groups of organisms. The 

 number of spinose forms is so great that it will be impossible 

 to give more than the briefest citation of a few of the leading 

 types, especially those which have come under the notice of 

 the writer ; on this account the number of examples derived 

 from the vegetable kingdom will be necessarily few. 



Am. Jour. Sci. — Fourth Series, Vol. YI, No. 33.— September, 1898. 

 17 



