32 



STUDIES IN EVOLUTION 



forces (nerve force, or neurism, and thought force, or phren- 

 ism) are not primary, and, although doubtless affecting 

 growth in higher organisms, cannot be original causes appli- 

 cable to all forms of life, both plant and animal. 



In tabular form, the divisions and relationships of the 

 factors of spine genesis may be expressed as follows: 



Spines originate by 



/■constructive agencies 

 (concrescence) acting ' 



destructive agencies 

 (decrescence) actini^ 



/■ ( extrinsically |^ from external 

 ( (centripetally) ) stimuli. 



B 



intrinsically / from growth 

 ( centrif ugally ) J force. 



extrinsically I 

 ( centripetal ly) ) 



from external 

 restraint. 



D 



( intrinsically ) from deficiency 

 \ (centrifugally) j of growth force. 



Under the last four divisions (A-D) it is proposed to 

 discuss the origin of spines, and from the observations made, 

 to derive certain conclusions regarding the significance of the 

 spinose condition. 



A. External Stimuli. 



Under external stimuli are included all the forces of the 

 environment (chemical, physical, organic, and inorganic) 

 which, through their impact or influence on an organism, 

 produce a consonant favorable change or disturbance. In 

 general, it will be seen that the number of impressions and 

 their power will depend largely upon the position and char- 

 acter of the surface upon which they impinge. The more 

 exposed the position, the greater will be their strength and 

 number, and if these stimuli or impressions are intermittent, 

 and not so violent as to produce waste and rupture, growth 

 will ensue. Under ordinary conditions, exposed parts will 

 naturally be the first to receive sufficient stimulus to produce 

 growth, and there will be normally a direct correlation be- 

 tween growth and stimulus. In a simple diagrammatic form, 



