48 STUDIES IN EVOLUTION 



the growth of matter at the intersections of lines, lamellae, 

 ridges, etc. Furthermore, it was indicated that this progres- 

 sive differentiation could be produced either (a) by the direct 

 action of external stimuli affecting the amount of nutrition 

 brought to a certain structure, (6) by the stimulus and dis- 

 persion of growth force, or (c) by a combination of the two 

 forces. In this differentiation of the features which are 

 generally called "ornamental," it will also be shown that the 

 spine is the final result of progressive differentiation, and, as 



? previously indicated, can be formed out of a variety of other 

 structures. The term " ornamental " is mainly one of human 

 interpretation, and is used simply in apposition to "plain" 

 or " simple ; " for example, a clam cannot be imagined as 

 consciously favoring a particular kind or arrangement of 

 tubercles for ornamental purposes. 



In a reticulate or cancellate surface formed by the crossing 

 of raised lines, ridges, or lamellae, it is evident that the 

 causes or forces producing such structures will be increased 

 at the points of intersection, and normally the amount of 

 growth will here be greatest. In this way it is possible 

 to account for the very common presence of spines at the 

 intersections of the radiating and concentric lines on many 

 Mollusca and other organisms. 



A few examples will now be given illustrating the differ- 

 entiation of various structures into spines. 



The points of intersections of the elements of the lattice in 

 the Radiolaria are where spines are most frequently found ; as 

 in Larnacalpes lentellipsis, Orosphcera Huxleyi, Carposphcera 

 melitomma, etc. 26 In Xiphosphcera pallas, the ridges about 

 the openings or meshes are granular, and the intersections 

 are raised into spines. Many of the discoid shells have their 

 edges differentiated into spines; as Heliodiscus asteriscus, 

 H. cingilluni) H. glyphodon, Sethastylus dentatus, Heliodry- 

 mus dendrocydus, etc. When an edge becomes elevated and 

 defined as a carina, this structure is also often spiniferous ; as 

 in Tripocalpis triserrata and Astropilium elegans. The final 

 differentiation of the radiate arrangement in the Radiolaria 



