1886.1 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 79 



(c) .The tliird case is one which is often (lil'lieult to distinguish from a 

 case of evolution or one of stable equilibrium for reasons presently to be 

 given. This case is often designated by the terms degeneration, degra- 

 dation, retrogressive development, retardation, and other like words and 

 phrases. But at the very outset we may be confronted by an inquiry 

 as to how such degeneration arose, and also be asked how w'e know 

 that it is an actual degeneration. A study of development indicates 

 that anatomists have often used the terms indic;itive of degeneration 

 hastily and iu an ill-considered sense. In order that my meaning may 

 be made clear it will be necessary to consider the possible ways in which 

 degeneration, real or apparent, may arise, and in this quest embryology 

 will be our best guide. In order to make our meaning the more di- 

 rectly applicable here the illustrations used will also be drawn from 

 studies upon fins and similar processes of the bodies of fishes. What 

 holds there is applicable as a general principle elsewhere. As it is, it 

 is evident that there are several types of degeneration, so called, some 

 of which cannot be properly included under the one same term. 



[aa) True or actual degeneration may be defined as that sort which is 

 witnessed when, for example, the preanal fin-fold of the Salmon is de- 

 veloped to the protopterygiau stage, with a row of actinotrichia on 

 either side, but is soon after absorbed so as to disappear coini)letely, and 

 long before the animal is fully developed. Another illustration is that 

 of the suctorial disk of Lepidosteus, which disappears in like manner, 

 leaving but very sWght traces of its existence in the adult. Such ^\ 

 method of degeneration, which involves the total atrophy of a stiuctnre, 

 embraces, for the most part, in the range of its action only so called 

 larval characters. This type of degenerative action is operative within 

 the life-time of an individual. 



{bb) The next subtype of apparently retrogressive development is 

 probably not actually retrogressive, if it is intended to apply the ex- 

 pression in its strictly literal meaning, but is only apparcMitly so, at 

 least in many cases. An instance of that is the " adipose lins" of fishes. 

 These have developed as far as to the stage represented by the fins of 

 the Dipnoi, but have been arrested at that stage and have advanced no 

 farther. It would therefore be pure hypothesis, uusui)ported by any 

 evidence whatever, to assume that that type of fin had been derived by 

 degradation from a dorsal in which there were wide interrndial inter- 

 spaces between true bony rays. Far rather let us snpiMjse that the 

 development has been so retarded in its advance toward the evolution 

 of the Teleostean type of fin as to preserve the older Dipnoan condition. 

 The term retardation, so often used by Coi)e, expresses the facts of the 

 case far better than to say degeneration. This applies, however, so far 

 as we can see, only to individual development, bejond which embry- 

 ology, it must be admitted, does not affoid anything more than hypo- 

 thetical clews, 



