i8o PRIMARY FACTORS OF ORGANIC EVOLUTION. 



an adolescent condition. Similar comparisons may be 

 made in the other genera. Progressively through each 

 series, the adult structure of any genus forms the last 

 immature stage of the next higher, until the highest 

 member in its ontogeny represents serially, in its stages 

 of growth, all the adult structures, with the larval and 

 immature stages of the simpler genera. It is evident 

 that in the identification of specimens belonging to 

 the Terebratellidae, whether recent or fossil, the strict 

 specific characters must be given first consideration. 

 Species, therefore, must be based upon surface orna- 

 ments, form, and color, within certain limits, and gen- 

 era only upon structural features developed through a 

 definite series of changes, the results of which are per- 

 manent in individuals evidently fully adult. 



In each line of progression in the Terebratellidae, 

 the acceleration of the period of reproduction, by the 

 influence of environment, threw off genera which did 

 not go through the complete series of metamorphoses, 

 but are otherwise fully adult, and even may show re- 

 versional tendencies due to old age ; so that nearly 

 every stage passed through by the higher genera has 

 a fixed representative in a lower genus. Moreover, 

 the lower genera are not merely equivalent to, or in 

 exact parallelism with the early stages of the higher, 

 but they express a permanent type of structure, as far 

 as these genera are concerned, and after reaching ma- 

 turity do not show a tendency to attain higher phases 

 of development, but thicken the shell and cardinal pro- 

 cess, absorb the deltidial plates, and exhibit all the 

 evidences of senility. 



