10 STUDIES IN EVOLUTION 



significance from an evolutionary standpoint may be confi- 

 dently determined. 



Grrowth 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 



12 3 4 5 



^l_ 



FienEES 1-5. — Different stages in tlie 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 

 FiGUKE 6. among animals and plants most spines conform 



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



atmg ridge of Often there are various kinds of surface orna- 



opondi/tus 



princeps-show- meuts, which by growth and differentiation de- 



ing the series ^ ■ i_ ■ -n i j_i ■ ij_ x- 



of flattened velop into spines. ay rhythmic, alternating areas 



spines. Qf 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 



