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



When, from any cause, the forces of nutrition are directed 

 toward spine production, and when the direct results are 

 accomplished in the reciprocal formation of one or more spines, 

 there is often an apparent inductive influence or impulse given 

 to growth toward the further production or repetition of 

 spines. This may result in the formation of compound spines, 

 or a group of spines, or even produce a generally spinous con- 

 dition. 



Naturally, spines arising through growth force may be use- 

 ful for defense and offense, and the selective influences of sex 

 and mimicry may also tend to greater development and elabora- 

 tion. Furthermore, growth forces reacting on any external 

 structures, as lines, lamellae, ribs, nodes, etc., may tend to dif- 

 ferentiate euch ornaments into spines. 



Therefore, under the general consideration of spines pro- 

 duced through growth force, the following factors are offered 

 for consideration : 



B. From. Groioth Force. 



B 1. — Prolonged development under conditions favorable for 

 multiplication. 



B 2. — By repetition. 



B 3. — Progressive differentiation of previous structures. 



B 4. — Secondary development through the selective influ- 

 ences of defense, offense, sex, mimicry, and other external 

 demands. 



C. External Restraint. 



Intermittent stimulus, as previously shown, produces growth 

 in the direction of function. When the growth equals the 

 waste, an equilibrium or static condition is reached, and no 

 relative change occurs. The absence of either extrinsic or 

 intrinsic stimulus will not be favorable to growth, and under 

 such conditions, an organ or structure may remain undevel- 

 oped, or, if already present in the organism, it may waste away 

 and degenerate into a vestigial structure, or even disappear 

 altogether. 



On the other hand, it is well known that continuous pressure 

 not only prevents growth, but in addition resorption takes 

 place, and in this way, the whole or a portion of a structure 

 may be removed. These changes have frequently been studied 

 in embryos, as well as in many internal structures, and are also 

 familiar in the enlarged pedicle openings of many Brachiopods, 

 caused by pressure of the pedicle, and in the similar opening 

 for the byssal plug of Anomia. Packard 54 gives examples 

 among the Crustacea and Insects, which- are clearly to the 

 point. He says of the Crustacea, '*' It 1nay here be noted that 

 the results of the hypertrophy and overgrowth of the two 



