431 



a. Catabatic stage, in which the individual loses its ephebic 

 characters. 



b. Hypostrophic stage, in which the continued loss of charac- 

 ters causes pronounced reversion. 



Examples and Remarks. 



It maybe noted that in an individual o f a progressive series Ge- 

 rontic changes are truly reversionary, the decline due to diminishing 

 vitality in the Catabatic stage causing a certain degree of reversion to 

 the characters of the Neanic stage. In an individual of a retro- 

 gressive series, however, in which the characters of the Ephebic 

 stage are less elaborate in development than those of the Neanic, or 

 even Brephic stages , the Gerontic stages shew simply characters of a 

 still further decline. In some extreme cases, however, the Hypostro- 

 phic stage of an individual of a retrogressive series may possess charac- 

 ters of, apparently, a renewed progressive development; from which 

 it would seem that when reversion has completed the cycle of changes, 

 further reversion may produce characters belonging to a recommence- 

 ment of the cycle. 



In applying the definitions of the Brephic and Neanic stages, it 

 should be remembered that specific characters sometimes make their 

 appearance in ontogeny before generic. The period at which any 

 character appears depends largely on the length of time for which it 

 has been a character of the race. In many cases , characters that are 

 regarded as specific have a higher antiquity than those that are regar- 

 ded as generic. 



It may also be pointed out that the physiological episodes of birth 

 and puberty have no definite relation to particular stages in the above 

 scheme , though they may accelerate or retard purely morphological 

 characters. 



As a simple example of the ontogenetic stages , we may take the 

 Ammonite now known as Deroceras ziphus, of which some good 

 figures were given by Wright under the name of Aegoceras Du- 

 dressieri. Confining our attention to the surface characters, we see, 

 following on the Embryonic protoconch, these stages : 



/Brephic, Neanic, Ephebic, Gerontic | ^^.^ ^^^^^^.^ -^ 



I Smooth, Costate, bpmous, Costate ; 

 clearly Catabatic with reversion to Neanic characters. It does not 

 happen, in this species, to be succeeded by a Hypostrophic stage; but 

 it is shown by closely allied species that the costate surface would be 

 succeeded by a smooth one , that is to say, a reversion to the Brephic 

 stage so far as this character is concerned. 



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