THE PATH OF DEGENERATION IN ANIMALS 187 



we have Seen the path of progressive evolution 

 being retraced, at least in the case of these reptiles. 

 The present state of knowledge does not permit an 

 extension of the enquiry to the conditions found in 

 higher animals. However, according to Flesch, 

 traces of a sensory epithelium have been found in 

 the pineal body of man, the horse, the sheep, and 

 the bat : that is to say, that in these creatures too 

 the oldest parts of the structure have resisted 

 degeneration longest. We cannot refrain from the 

 conclusion that in this series degeneration retraces 

 to a large extent the steps of original advance. 



2. Degeneration of the organs of sight in deep- 

 sea Crustacea. We cannot however establish the 

 conclusion of the last paragraph as a general 

 principle. 



The fauna of the deep sea includes a large 

 number of Crustacea, and in these the eyes, which 

 are relatively useless, are often degenerate. The 

 course of the degeneration is generally definite, and 

 of all the structural parts the most long-lived are 

 the eye-stalks, although we know that these are 

 a recent formation. A number of examples chosen 

 from Decapod Crustaceans, which are specially 

 abundant, will illustrate this point. 1 



Nephropsis, which lives in the Atlantic and 

 Indian Oceans at moderate depths, is a relative of 

 the Lobster. The optic stalk is short and carries 



1 See Pelseneer, V Exploration zoologique des mers profondes 

 (Conferences Universitaire de Bruxelles). 2 Annee, 1890. 



