362 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



pentaclactyle limb of the higher Vertebrata has been 

 developed, we must suppose that the greater number of 

 the cartilaginous pieces have undergone reduction, and 

 that, in the Ichthyosauria for example, or in the frog, 

 where there is a rudiment of a sixth digit to the foot, the 

 number five is exceeded in consequence of the re- 

 duction not having been definitively impressed on the 

 organism by inheritance ; on the other hand, the 

 possession by a Mammal (e.g. man) of more than five 

 digits (polydactylism) must be regarded rather as an 

 abnormality than as a return to an ancestral condi- 

 tion, and this because the gap between a man and a 

 polydactylous ancestor is too wide for us to be able 

 reasonably to believe in an " atavism " so far-reaching. 



In the Ganoidei or Teleostei, the pterygial portions 

 of the fins are reduced, but the reduction is atoned 

 for by the replacement of the horny fibres by osseous 

 tissue. In Ceratodus the fin takes the form of a 

 central axis of cartilaginous pieces, with rays on both 

 sides ; and in Protopterus it becomes filamentar, owing 

 to the loss of the lateral rays. Gegenbaur regards the 

 fin of Ceratodus as the most primitive arrangement 

 (archipterygium) ; but, as Balfour has pointed out, this 

 view of the matter is opposed by the facts that in 

 Elasmobranchs there are indications of rays on one 

 side only of the basipterygium, and that the support- 

 ing bar is, at first, basal, and not central. 



Like the limbs of higher Vertebrates, the fins of 

 fishes are, at first and in most cases, locomotor in 

 function, wherein they are aided by the tail ; just as 

 the former are supporting organs, so, too, are the fins. 

 This may be seen by removing the fins of one side, 

 when the fish falls on to that side ; or by cutting off 

 both pectorals, when the body inclines forwards and 

 downwards. In mud-dwelling fishes the pelvic fins 

 are rudimentary or absent, disuse producing degrada- 

 tion. One of the most remarkable modifications of 



