ORIGIN AND SIGNIFICANCE OF SPINES O 



As these developed, the spinose forms became more abundant 

 until after the culmination of the group is reached, when this 

 tj'pe either became extinct or was continued in smaller and 

 less specialized forms. In applying this principle to any 

 order of plants or animals, several precautions are necessary. 

 The estimate must be based approximately upon the general 

 average of the totality of specific characters, whether a genus, 

 family, order, or even a class is being considered. A short- 

 lived family or genus, or the terminal members of specialized 

 groups, therefore, cannot be taken as representing the develop- 

 mental status of the larger divisions, because they culminated 

 and disappeared independently of the culmination of the class 

 to which they belong. On a small scale, however, each 

 epitomizes the rise and decline of the larger group, and the 

 principles of correlation commonly applied in ontogeny and 

 phylogeny can likewise be used in the study of spines and 

 spiniferous species, with equally exact results, whenever the 

 principal factors are understood. 



Law of Variation. 



Before undertaking any general or special examination of 

 the life histories and interpretation of spinose organisms, it 

 is desirable to consider briefly some of the biogenetic prin- 

 ciples which are considered to bear directly on the problems 

 here under discussion. 



First among these is the law of variation or change, which 

 is so generally recognized as to require but the briefest 

 restatement. 



The organic as well as the inorganic world is subject to 

 all the forces of nature, internal and external, molecular and 

 molar, and even a partial stability is gained only through a 

 regulated adjustment. In organisms this change is momen- 

 tary and persistent, while in most inorganic substances it is 

 slow and intermittent. The results of this continual read- 

 justment constitute modification, which may be progressive 

 or regressive, continuous or discontinuous (in the sense of 

 accelerated, uniform, or retarded). They are everywhere 



