THE THREE MODES OF DEVELOPMENT. 37 



The accomplished author of the " Paleontologie Francaise " denied that the 

 internal parts were affected by old age, "ne montrent qu'une complication 

 toujours croissante et jamais de degenerescence." This error was corrected by 

 Quenstedt who pointed out that the closer approximation of the sutures in large 

 individuals was due to senility, and the author is now able to record that he has 

 either observed or seen figured similar cases of approximate sutures, indicating 

 senile degradation in all of the different forms of chambered shells, except in 

 Belemnoids, which have not yet been investigated. 



D'Orbigny and Quenstedt were both satisfied with noting the details of 

 the old stage in the individual, the diseased aspect of certain forms, and their 

 reproduction of the characters of their own young and of those of older forms, 

 but did not attempt to explain the wider meaning of these parallelisms. In 

 former papers we have asserted that the close similarity between the smooth, 

 straight Baculites of the Cretaceous, the extreme nostologic form of the Ammo- 

 noids, and the smooth, straight Orthoceras of the Cambrian, the common radical 

 from which both Ammonoids and Nautiloids sprang, is parallel with the resem- 

 blances which exist between the nostologic or oldest stages of the individual and 

 its own young. 



This resemblance between radical and geratologous forms in smaller groups, 

 like the Arietidae, was slighter, and often consisted merely in the smoothness 

 of the shell, or loss of the keel, or decrease in the amount of involution of 

 the whorl. Ast. Cottenoti and Psil. planorbe both have the compressed helmet- 

 shaped outline of the whorl in section, and are smooth, though Cottenoti exag- 

 gerates this form, or is more involute and natter than planorbe. The most 

 remarkable cases of -geratologous reversion in the Lias are found in Oxynoticeras 

 Lotharingum, in which the old whorl loses its keel, and exactly reassumes through 

 degeneration the compressed helmet-shaped aspect of the adults of Psil. planorbe. 

 Even this extreme example among Arietidas, however, is not in any sense 

 an uncoiled shell. It is very nearly a complete parallel with the smooth 

 keelless proximate radical form Psiloceras, and may therefore be termed a noso- 

 logic species. It barely attains this extreme rank in degeneration, whereas other 

 species, such as Ast. Cottenoti, which retain the keel and do not decrease in size of 

 whorl, are only clinologic approximations. 



The resemblances which occur between the young and old of the same indi- 

 vidual in the same parts and organs take place because the organs lose their 

 power to exercise the functions which distinguished them in the adult, and 

 becoming useless, are either partly or wholly atrophied and resorbed. 



The Three Modes of Development. 



The likeness between the younger stages of growth and the senile stages 

 of decline in the same individual is, as we have just shown, due to the disap- 

 pearance in old age of the specialized ephebolic characteristics acquired in the 

 progressive nealogic and adult stages of growth. In groups the resemblances 

 between radical and geratologous forms is occasioned by a similar suppression in 



