ORIGIN AXD SIGXIFICANCE OF SPIXES 33 



this would be expressed by a series of lines, the first repre- 

 senting a plane surface. Then, owing to the impossibility of 

 maintaining a uniformly intermittent stimulus or a uniform 

 response, some point or spot on this surface would grow in 

 excess of the others. This difference would be augmented 

 by the more favorable position of the spot to receive stimuli, 

 further growth would take place, the growth force decreasing 

 with the increase of distance, and the final action of these 

 forces, stimulus and growth, would be to produce a pointed 

 elevation. Such structures or outgrowths, especially when 

 made of hard rigid tissue, would be termed spines under the 

 general definition. The spine may be viewed as an attached 

 organism, and its conical habit of growth would then con- 

 form to the law of radial symmetry, as determined by the 

 physiological reaction from equal radial exposure to the 

 environment. That all the irregularities of contour in all 

 organisms have not developed into pointed processes or 

 spines is not, therefore, the fault of the simple reciprocity 

 between growth and external stimuli. This kind of develop- 

 ment, however, requires a direct and immediate responsive 

 external growth to the exciting force, which from various 

 causes is frequently absent. Obviously, stimuli which result 

 simply in motion or equivalent internal adjustments can have 

 no effect toward spine production, so that only the results of 

 such stimuli as bring about some accompaniment of super- 

 ficial growth will be considered. 



With the exception of perfectly spherical, freely moving 

 forms, all organisms have certain parts which are more 

 exposed to the forces of the environment than others, and 

 from the principles already enunciated, such exposed parts 

 under normal conditions will grow. This growth in the 

 direction of function and stimulus, when acted upon by 

 the hereditary functional and structural requirements of the 

 organism, serves to produce the various external organs and 

 appendages. But when the surface upon which the stimuli 

 fall is not thus predetermined by heredity to grow into a 

 certain organ or functional part, there results a normal 



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