vii EVOLUTIONAKY OKIGIN OF LIMBS 449 



The muscularization of the jugular pelvic fin of Teleostean fishes 

 is admittedly secondary : the limb rudiment becomes muscularized 

 by the myotomes to which it happens to be opposite at the time 

 muscularization begins : but if this fact be admitted it is not open 

 to us to deny the possibility of a similar process having taken place 

 in the successive positions taken up by the pelvic limb in the course 

 of the movements which it has undergone during phylogenetic 

 evolution. 



A further objection urged against the Gegenbaur hypothesis is 

 that there have not been discovered, up to the present, any examples 

 of the intermediate stages between gill-septum and limb which must 

 have existed if this hypothesis be a true theory. This objection 

 appears to be a valid one. 



Again it is urged that in those Vertebrates which would appear, 

 in this respect, to have retained most nearly the primitive condition 

 (Cyclostomata, Elasmobranchii) the gill-septa are fixed firmly in 

 position and are therefore not likely to become converted into motor 

 organs, which must necessarily project beyond the surface and be 

 freely movable. This objection like the last appears to be a valid 

 one. 



It will be apparent from the short sketch which has been given 

 of the two rival views of the evolutionary origin of the limbs of 

 Vertebrates that neither can be regarded as wholly satisfactory. 

 However these hypotheses are old, as the science of Embryology goes. 

 They were designed to fit the data available at the time they were 

 formulated and the -great bulk of subsequent work upon this 

 particular problem has consisted in the adducing of new facts which 

 appear conveniently to fit on to those already accumulated by the 

 supporters of one view or the other. In a rapidly advancing science 

 like Embryology however it is advisable to have from time to time a 

 stocktaking of the facts of contemporary knowledge with the object 

 of seeing whether the more extensive body of available facts suggests 

 the same working hypotheses as were suggested by the facts known 

 at earlier periods or, as is always possible, something quite different. 

 The putting this principle into practice is more conducive to progress 

 and more stimulating to research than the mere accumulation of 

 further facts to support or to confute the working hypotheses of 

 earlier times. 



The External Gill Hypothesis. — Applying this principle to 

 the problem of the evolutionary origin of the limbs one finds an 

 important set of data which were not available to Gegenbaur or 

 Balfour. In their day there was no proper appreciation of the 

 importance of the fact that there existed in three of the less specialized 

 groups of Vertebrates— Urodele Amphibians, Lung-fishes and Cross- 

 opterygians — those organs which have been described in Chapter III. 

 under the name External Gills. The mode of development of these 

 organs is now known in all three of the groups mentioned and the 



VOL. II 2 G 



