G E. Beecher — Origin and Significance of Spines. 125 



Art. XII. — The Origin and Significance of Spines: A 

 Study in Evolution ; by Charles Emerson Beecher. 



[Continued from page 20.] 



Categories of Origin. 



As previously shown, spines are formed either by growth or 

 by suppression, and therefore the processes determining their 

 production are either constructive through concrescence or 

 destructive through decrescence. Each of these is in turn 

 determined by forces from without the organism (extrinsic) or 

 by forces from within (intrinsic). In this connection, it is of 

 no especial moment whether or not the intrinsic forces are 

 primary or are an immediate or subsequent reflex from the 

 extrinsic. The main thing is the direction of the dominant 

 force, whether centripetal or centrifugal. If, in some cases, it 

 can be shown that spine development has been accomplished by 

 intrinsic forces in the organism, then this development may be 

 brought about independently of the environment and possibly 

 at variance with it. Also, if in other cases, the extrinsic forces 

 or the influences of the environment have caused spine growth, 

 it may in some instances illustrate the formation and transmis- 

 sion of an acquired character, or at least the operation of 

 organic selection. 



The point has now been reached where it is impracticable to 

 make a rigid classification of the direct factors or an exact 

 determination of primary and secondary causes. It was 

 remarked at the beginning of this paper, that single causes 

 were not sufficient in every case to account for spine growth, 

 and while it is comparatively easy to formulate abstract expres- 

 sions or terms covering all possible cases, it will be found 

 difficult to construe properly certain factors to fit into any 

 particular conception. In illustration of this, the foregoing 

 statements may be taken. Thus, spines are formed by the 

 only means possible, either by growth of new tissue or by 

 decrease in old. Again, the forces must act from the interior 

 or from the exterior; in other words, they must be intrinsic or 

 extrinsic. But in some specific instance, while considering 

 food, forces of nutrition, external or internal demands, reac- 

 tions, etc., a question may arise as to the proper disposition 

 to make of a spine developing primarily by external stimuli 

 and becoming a defense and secondarily a weapon ; yet which 

 by differentiation in time loses some of its protective and 

 offensive qualities, and by selection may be confined to one 

 sex. 



