20 GENESIS OF THE ARIETIDA. 
in other animals, especially in man himself, the decline was marked by degra- 
dation of certain characters, and the number of parts undergoing degeneration 
was gradually increased, until finally the whole of the body was more or less 
affected. 
This period has been frequently described by the author in previous publi- 
cations, and will be more fully described farther on. It is necessary now only 
to call attention to the fact, that the geratologic or old-age period can be natu- 
rally subdivided into two quite distinct stages. The first, or Clinologic stage, 
included the retrogressive transformations during which the nealogie and ephebolic 
characters became resorbed one after another, usually in reverse order to the succession in 
which they were introduced during the progressive stages of growth. The size of the 
whorl also, sooner or later according to the species, showed retrogression during 
this period. All of these retrogressive tendencies reached their extreme ex- 
pression in the last and final stage of the ontogeny of the individual. In this 
stage the spines, pile, and often the keel and channels, when present, were 
lost, and the size of the whorl was so much reduced in all its diameters that it 
became more or less rounded, whatever the angularity of the whorl during the 
ephebolic period. This stage we have designated by the term Nostologic, on 
account of the likeness to its own nepionic period, which was finally acquired by 
the smooth, almost rounded whorl after the loss of its progressive characters. 
Geratology, or the study of the relations of these old-age stages, shows, as we 
shall try to demonstrate farther on, that the clinologic characters can be used to 
predict the degradational modifications which appeared in any series of orna- 
mented shells when placed under such unfavorable conditions that their descend- 
ants became degraded, and series of more and more retrogressive forms were 
gradually brought into existence. A number of such series have been traced by 
several authors, and they usually end with a perfectly straight form. This form 
terminated the phylogeny of the series in a manner comparable to that in which 
the nostologic stage terminated the ontogeny of the individual. It is usually 
separated also by a gap from all other species, which has not yet been fully filled 
by intermediate species. This nostologic adult form, the so-called genus Baculites, 
is not only comparable in this way and by means of its smooth and compressed 
cylindrical whorl with the last stage of ontogeny, but it is also a very complete 
reversion to the aspect of the earliest radicals of its own class, the Orthoceratite 
and Endoceratite. 
This nomenclature is similar to that originated and published by Haeckel, and 
at first sight may appear to many naturalists as identical; but it is really only 
complementary. It is based upon strictly structural and morphological grounds, 
whereas Haeckel's nomenclature’ was entirely physiological. This eminent author 
regarded the ontogeny of an individual as a cycle divisible into three periods ; 
first, the progressive stages of Anaplasis, or those of progressive evolution; sec- 
ondly, the stages of fulfilled growth and development, Metaplasis ; thirdly, those 
of decline, Cataplasis. He also appreciated and gave full weight to the general 
physiological correlations which are traceable between the history of a group and 
1 Morphol. d, Organismen, II. pp. 18-23. 
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