10 STUDIES IN EVOLUTION 



significance from an evolutionary standpoint may be confi- 

 dently determined. 



Growth of a Spine. 



The growth of a spine is either direct an . progressive, or 

 indirect and regressive. It is direct when it is developed by 



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FIGURES 1-5. Different stages in the growth of a spine. 1, plane surface ; 

 2, slight elevation ; 3, node ; 4, short spine ; 5, completed simple spine. 



the addition of new tissue. In this way growth is attained 

 in the antlers of a Deer, the horns of a Cow, the ordinary 

 spines of Brachiopoda, Mollusca, and Crustacea, 

 and in other similar examples covering the major- 

 ity of cases. Growth is indirect, however, when 

 the spine represents atrophy or suppression of an 

 organ through the loss of its accessory parts ; as 

 in the thorns of the Locust and the Barberry, 

 the spiniform termination of the stems of the 

 Pear, or the spurs on the Python. 



The direct development of a spine is essen- 

 tially the same process in all cases. At a given 

 point on the surface of an organism, there first 

 appears a slight elevation, which becomes higher 

 and higher, and is usually conical in form. This 

 cone represents the simplest type of spine; and 

 FIGURE 6. among animals and plants most spines conform 

 a single radi- to this primitive pattern (figures 1-5). 



Often there are various kinds of surface orna- 



w- ments, which by growth and differentiation de- 

 of g Ha^ned velop into spines. By rhythmic, alternating areas 



spines. O f accelerated or retarded growth, the concentric 



laminae on many molluscs may produce spines, as shown in 

 figure 26. In the same way the radiating ridges may be 



