ORIGIN AND SIGNIFICANCE OF SPINES 101 



varietal value, and are rated of specific, generic, and some- 

 times of family rank, or even higher. They;hay^;^yefore{ 

 acquired a fixed importance in these special groups, and ar<e 

 recognized in the same categories with ^pJiysroiogiyaL j s^ J 

 structural characters. The differences which appear at an 

 early period in higher genera are the bases of distinction 

 among lower genera. If the spines or other similar fea- 

 tures do not make their appearance in an individual until 

 a late adolescent stage, they are usually of negative value in 

 a scheme of classification. This agrees with the general 

 principle recently suggested by Harris, 32 that when the 

 main features of the ornament (= spines, etc.) are fore- 

 shadowed in the larval and early adolescent stages, they are 

 to be regarded as of taxonomic value. 



The preceding diagram illustrates the previous statements, 

 and shows the correlation between the stages and conditions 

 of growth in the ontogeny of a spinose individual, with its 

 phylogeny, and also the chronology of groups containing 

 spinose forms. The numbers indicating chronology simply 

 refer to successive periods of time. In particular cases they 

 may be long geologic ages ; as Cambrian, Ordovician, Silurian, 

 Devonian, and Carboniferous, or in other instances they may 

 represent much shorter periods. 



From the study of the ontogenies of spinose forms, it has 

 already been ascertained that they were simple and inornate 

 during their young stages ; and from the phylogenies of the 

 same and similar forms, it was likewise learned that they 

 were all derived from non-spinose ancestors. It has also 

 been shown that spines represent an extreme of superficial 

 differentiation which may become fixed in ontogeny, and 

 the further conclusion, that spinosity represents a limit to 

 morphological and physiological variation, has been reached. 

 Finally, it is evident that, after attaining the limit of spine 

 differentiation, spinose organisms leave no descendants, and 

 also that out of spinose types no new types are developed. 



