C. E. Beecher — Origin and Significance of Spines. 131 



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 pre- 

 determined by heredity to grow into a certain organ or func- 

 tional part, there results a normal responsive action between 

 growth and stimulus, which, as already seen, tends to produce 

 a conical or spiniform growth. 



Under ordinary favorable conditions, simple external 

 stimuli acting blindly through no agencies of selection would 

 develop spines on all the most exposed parts, and tend to dif- 

 ferentiate ornamental features. This has been the case with 

 many organisms and colonial aggregates possessing no power of 

 selection or not acted upon by any forces of determination, 

 conscious or unconscious. In such cases, spines may or may 

 not serve for protection, and their function, if any, can be 

 only determined separately for each case. If, however, the 

 added function of offense is included, it is manifest that 

 the spines must be located in special positions adapted to use 

 for offensive purposes as on the tails of some animals and 

 not necessarily over vulnerable parts. Here the selective 

 agency of special adaptation is shown. Again, if while there 

 is agreement in other essential characters, spines or 

 horns are confined to either sex, it is evidently a case of 

 sexual selection. Further, if they develop in harmony with 

 the environment, or in a manner parallel to similar features of 

 other organisms, it is through the operation of physical selec- 

 tion. 



Altogether, under the general forces of external stimuli, 

 there are five aspects in which to consider the production and 

 growth of spines, viz : 



A. From External Stimuli. 



Al. — In response to stimuli from the environment acting 

 on the most exposed parts. 



A2. — As extreme results of progressive differentiation of 

 ornaments. 



A3. — Secondarily as a means of defense and offense. 



Ai. — Secondarily from sexual selection. 



A5. — Secondarily from mimetic influences. 



B. Growth Force. 

 In unicellular organisms, growth force, or bathmetic energy, 

 must reside wholly in the germ cell, and therefore is con- 

 cerned with reproduction as well as with cell differentiation. 

 In multicellular organisms, the growth force is in both germ 

 and soma cells, and its relative strength seems to depend upon 



