14 STUDIES IN EVOLUTION 



The first type mentioned develops horns or spines by 

 simple increscence (figure 15); for example, the Ox: the 

 second, by successive replacement (figure 16); as in the 

 Deer: the third, by serial repetition (figure 17); for example, 

 Spondylus : the fourth, by decrescence (figure 18); for 

 example, the Barberry. 



Localized Stages of Growth. By the multiplication of sur- 

 face ornaments through the process of interpolation, many 

 Mollusca present stages of spine development in two direc- 

 tions. (1) The normal series is represented by the succession 

 of spines along a single sector of growth. For instance, in 

 the radial plications of a Spondylus or Lima, the earliest and 

 primitive spines are found near the beak, while those on the 

 ventral border of an adult specimen are the latest and most 

 highly developed (figure 30). These successive stages, there- 

 fore, are in the direction of growth, and may be called longi- 

 tudinal. (2) By the radial divergence of the ribs or plications 

 and the interpolation of additional ones at various intervals, 

 as many transverse compound series of spines finally appear 

 along the periphery as there are primary radii. Hence, in a 

 given case, there may be two radii continuing to the beak, 

 then by interpolation there are successively 5, 11, 23, etc., 

 radii, the highest number being found at the periphery 

 (figures 19, 20). Moreover, by taking the distal spines on 

 these 23 rows, there result the same stages of spine develop- 

 ment as shown in the longitudinal series along any primitive 

 plication (figure 20). A pelecypod shell like Spondylus is 

 here used to illustrate this process, but the application may 

 also be made to the Brachiopoda as well as to the conical 

 non-coiled Gastropoda. In a coiled form like a cephalopod 

 or an ordinary gastropod, the longitudinal lines would follow 

 the whorls spirally, and the transverse lines would corre- 

 spond to the lines or increments of growth of the shell. 

 Species in which the radii are all introduced at an early 

 stage of growth (many species of Cardium, Pecten, Lima) 

 or in which the radii multiply by regular dichotomy would 



