28 GENESIS OF THE ARIETIDA. 
We reproduce this conclusion in full, though, as may be seen by reading the 
preceding pages, we differ essentially as to the causes that produced parallelism 
between different series in the same or different localities. Nevertheless, Waagen 
agrees with us in rejecting the doctrine of natural selection as a fundamental 
cause of parallelism, and has also stated in 1875, from independent observations, 
the possibility of doing what we have been putting in practice ever since 1866; 
namely, predicting what sort of species would be found as descendants of certain 
given forms, and then subsequently finding them. This experience has also 
been shared by Professor Cope, who makes similar statements of his own obser- 
vations among fossil and recent Batrachians and Reptiles. ‘The method pursued 
by both of us differs from that ordinarily used by naturalists in predicting the 
existence of new forms, in that it relies upon the action of the law of accelera- 
tion, and the constant recurrence of similar forms in different series of the 
same stock, or, as we have explained above, upon the law of morphological 
equivalence.’ 
Turory or Rapicats AND MorpnoLoaicAL EQUIVALENCE IN 
RETROGRESSIVE Forms. 
There are certain species among complicated acmic forms which became the 
ancestors of uncoiled degenerate series, that can be properly termed nostologic 
forms on account of their complete reversion to the uncoiled forms of the radical 
groups among Nautiloids. These were not confined to any special class of forms, 
though more frequent among the higher than among the trunk stock of radical 
forms. They are what we have called geratologous radicals. Thus Lobites of 
the Trias must have sprung from some geratologous radical among the Goniati- 
tine; and Hauer’s Cochloceras with its turrillites-like whorls, and the straight 
Rhabdoceras, both have sutures which indicate derivation from some genus like 
Helictites or Choristoceras among Ceratitinse of the Trias, ribbed shells with very 
simple sutures Choristoceras, also, had discoidal species in the Rheetic beds. 
We treat these forms as probably degradational, because of their simpler 
ornamentation and sutures, and also because the similar uncoiled shells among 
Gasteropoda and among Ammonitine may be followed until they grade into 
closely coiled and more complicated shells, from which they probably arose.’ 
The geratologous forms have a most important bearing on the conclusions reached 
in this essay. They terminate the geologic history of their suborders, just as the 
Turrillites and Baculites, and others, appear as the final forms of Ammonitine. 
They were also coextensive with the existence of the cephalopod type, and were 
evidently liable to be evolved at any time in their history, and to increase in 
1 The law of acceleration and of morphological equivalence has been stated in the Preface, pp. iv. and v. 
2 These lines were written before Zittel’s superb work, ‘‘ Handbuch der Paleontologie,” had appeared, 
in which (p. 431) he associates these forms in exactly the same order. Although his text does not allude to 
the genesis of the forms, his mode of arrangement shows that he probably had the same idea in mind. 
8 Parallelisms of Individuals and Order among Tetrabranchiate Mollusks, Mem. Bost. Soc. Nat. Hist., 
J., 1866-67, and Proceedings of same, I., 1866, p. 802. Genetic Relations of Stephanoceras, Proc. Bost. 
Soc. Nat. Hist., 1876, XVIII. p. 380. Also Genesis of Tertiary Species of Planorbis at Steinheim, p. 8. 
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