ORIGIN AND SIGNIFICANCE OF SPINES 93 



Categokies of Interpretation 



Having thus far examined the factors governing the origin 

 of spines, and found that they could be grouped into a 

 number of distinct categories, it is now desirable to interpret 

 these results, and endeavor to arrive at the real significance 

 of the spinose condition. 



The two main generalizations which will be discussed are, 

 first, that spinosity represents the limit of morphological 

 variation, and, second, it indicates the decline or paracme of 

 vitality. 



Spinosity a Limit to Variation. 



A number of data have already been given, leading to the 

 belief that, on becoming spinose, organisms have reached a 

 limit of morphological variation. They may continue to 

 develop more and more differentiated and compound spines, 

 but no new types evolve out of such a stock. 



The subject may be treated in two ways, both leading to 

 the same conclusion. First, the stages and processes involved 

 in the growth of a spine itself may be studied, and next the 

 development of spines in the ontogenies and phylogenies of 

 animals and plants may be examined. 



The growth of a spine has already been described, and it 

 was shown that this type of growth may arise from speciali- 

 zation of other ornamental features, such as nodes, ridges, 

 and lamellae, and also from the decadence of leaves, legs, etc. 

 These observations and numberless others which could be 

 made, will be sufficient to show that almost any kind of 

 superficial structure, as knobs, tubercles, ridges, lamina), 

 reticulations, etc., has by differential growth been changed 

 into spines; also, that organs of various kinds, as legs, 

 branches, leaves, etc., have by atrophy been reduced to 

 spines. In each case the parts in their development pass 

 through the various intermediate stages, and clearl}^ show 

 that the spine is a result and not a mean. Moreover, none 

 of these structures or organs are developed through the 



