80 FIN-RAYS IN CLASSIFICATION OF FISHES. 



In many cases degradation of some structure has, however, certainly 

 occurred. xVmost notorious case of tliis kindisthatof the Whales, which 

 have lost their functioual hind limbs. On investigating the condition 

 of the vestiges* of these limbs we find that the skeletal parts have 

 actually been arrested, as to the extent of their development, at a point 

 corresponding to an early embryonic stage, beyond which they fail to 

 advance. We are now ready to ask where this curtailment of develop- 

 ment began, and we find that there is logically no way out ot the difli- 

 culty except to admit that the retrogressive metamorphosis must have 

 begun after birth in each and every one of the series of individuals con- 

 stituting the race, because there could be no equilibration between ex- 

 trinsic forces on the one hand and intrinsic or organic forces on the 

 other as long as the foetus was protected and incapable of free move- 

 ment in titero. I see no escape, therefore, from the conclusion that the 

 second sort of degeneration^ designated by the word retardation for ein- 

 bryological reasons, has actually arisen in many instances through a 

 very slowly acquired undoing \)f development or loss of parts through 

 an extended series of adults which have as slowly transmitted theses 

 increments of loss or degradation, which so far as we can see is now 

 tolerably stable, though far less so than the development of functioual 

 structures, as has been shown by the researches of Struthers. 



The distinction between the type of degradation under aa and that 

 under hb is that iu the first the atrophy or loss of a part is an ontogenetic 

 process, while in the other the degree of degradation of a part is ac- 

 quired and becomes fixed hy hereditary transmission, and is therefore 

 presumably a phylogenetic process, because we see no evidence of any 

 gradual atrophy of such parts in the course of the ontogeny of the 

 animal. 



Another illustration of the use of the method of embryology is found 

 amongst the Catflshes. The genus Notiirvs has the adipose dorsal fin 

 adnateand^ontinuouswith the caudal, while in the genus Amiurus there 

 has been a decided advance upon the former arrangement, for the 

 reason that in it the adipose dorsal is separated by a wide interval 

 from the caudal. Amiurus, during its development, actnally recapitulates 

 very closely the stage represented by Noturus, which indicates that tlie 

 latter is taxonomically lower in rank than the former. And just in this 

 instance we also have a very good illustration of a principle of develop- 

 ment which ought to make anatomists cautious in the use of the word 

 " degeneration." While I see no evidence whatever of degeneration in 

 Noturus, I do see very obvious evidence of arrest or retardation of 

 development of its adipose fin at a point corresponding to a transient 

 stage in Amiurus. That is to say, this fin iu Noturus has been perma 



* Structures which are disappearing should be called rest'ujea. Structures which 

 are still imperfect but are appearing ought to be called rudiments. As it is, the word 

 rudiiueut is usually misapplied, so far as coucerus its literal sense, Avhcn speaking ojf 

 '• rudimentary organs." Such a distinction it seems to rao is important. 



