ORIGIN AND SIGNIFICANCE OF SPINES 99 



features were the chief variables. The stimuli which, during 

 the early life history of a group, were expended in internal 

 or physiological adjustments, later produce external differen- 

 tiation, and in this differentiation spinosity is the limit. The 

 presence of spines, therefore, indicates the fixity of the primary 

 physiological characters, together with the consequent inability 

 of the organism to change due to its decreasing vitality. 



Conclusion. 



Just as all the features of terrestrial topography are in- 

 cluded between the limits of plains and mountains, and the 

 mountains are considered as the limit of progressive ac- 

 cidentatiou, so the spines of animals or the monticules and 

 pinnacles of their surface may be considered as the limits of 

 progressive differentiation. The primitive base level, or 

 peneplain, becomes elevated, and by erosion is cut up into 

 tablelands, mesas, and buttes, with intersecting valleys. 

 The valleys are gradually deepened, and the country becomes 

 rougher until a maximum is reached. Then follows a reduc- 

 tion of the inequalities of the surface, and finally, in old age, 

 the smooth, gently rounded outlines of geographic infancy 

 again appear. So in organisms the smooth rounded embryo 

 or larval form progressively acquires more and more pro- 

 nounced and highly differentiated characters through vouth 

 and maturity. In old age it blossoms out with a galaxy of 

 spines, and with further decadence produces extravagant 

 vagaries of spines, but in extreme senility comes the second 

 childhood, with its simple growth and the last feeble infantile 

 exhibit of vital power. 



The history of a group of animals is the same. The first 

 species are small and unornamented. They increase in size, 

 complexity, and diversity, until the culmination, when most 

 of the spinose forms begin to appear. During the decline 

 extravagant types are apt to develop, and if the end is not 

 then reached, the group is continued in the small and un- 

 specialized species which did not partake of the general 

 tendency to spinous growth. 



