IX.] RETROGRESSIVE DEVELOPMENT IN NATURE. 9 



organs of special sense, although cases of the disuse of these 

 are of less frequent occurrence. Thus the caecilians, tropical 

 worm-like or snake-like amphibians, living underground, have 

 lost not only the sense of sight, but that of hearing also. They 

 possess neither tympanum nor tympanic cavity, and although 

 the auditory vesicle, which is buried in the interior of the 

 skull, still exists, the auditory nerve, which should be in con- 

 nection with it, supplying its sensitive nerve-endings, has 

 entirely disappeared. The sense of hearing must have become 

 useless to them in their life underground, or the organ would 

 not have degenerated ^. They are compensated for the want 

 of it by a remarkably keen sense of smell, which is more highly 

 developed in these animals than in any other vertebrates. 



Instances are also known of disuse causing degeneration in 

 the sense of smell ; thus the whales and dolphins have more or 

 less completely lost this organ which is so highly developed in 

 the rest of the Mammalia. 



Retrogression is, however, not always carried so far as to do 

 away with a structure altogether, although this generally 

 happens with the organs of sense, because they can scarcely 

 be adapted to other uses. But not infrequently the degenerat- 

 ing organ can be turned to account in some other way, and then 



^ It is now known that the above statements as to the existence of a 

 rudimentary auditory organ in Caecilia are erroneous. Recent researches 

 have shown us that these animals not only possess a complete auditory 

 apparatus, but that it is even more perfect than in other Amphibia. In 

 their splendid ' Ergebnisse zoologischer Forschungen auf Ceylon,' Heft 4, 

 1890, the cousins Sarasin have given an accurate account of the auditory 

 organ of a caecilian {Epicrumi \, and show that it is very far from being in 

 a degenerate condition. It possesses all the essential parts, the auditory 

 nerve is even larger than usual, and one of the ' maculae aciisikae ' present 

 is unrepresented in other Amphibia. These writers even prove that, in 

 addition to the ordinary apparatus, many accessory auditor3'' organs are 

 present in the skin, each of which contains an otolith : these are homo- 

 logous with the ' 01 gans of the lateral line ' of other Amphibia and of fish. 



Up to the present time our knowledge of the auditory organ of Catcilia 

 has been founded upon the statements of two excellent observers, Pro- 

 fessors Retzius and Wiedersheim ; but the material at their disposal was 

 restricted to a few badly preserved specimens. 



We must therefore maintain that the organ of hearing as well as that 

 of smell has been especially developed in the caecilians as a compensation 

 for the want of eyesight. Those conditions of life that would render the 

 power of hearing useless do not appear to exist. As a result of these 

 recent researches, I am now unable to adduce an example of a rudi- 

 mentary auditory organ. — A.W., 1891. 



