74 PRIMITIVE ANIMALS 



all the principal parts can be homologized with 

 corresponding parts in the gills of fishes. The sub- 

 sequent development of the head in this region takes 

 place by a gradual alteration of the gill-structures 

 into various organs associated with the ear and throat, 

 which finally in the adult subserve totally different 

 functions from those which they were originally 

 fashioned to perform. It may seem strange that it 

 has sometimes been argued that this does not imply 

 that the ancestors of these land vertebrates were fish- 

 like creatures and breathed by functional gills. It is 

 true that if we were desirous of exhibiting an excess 

 of caution we might argue that it is only safe to 

 conclude that the ancestor of the land vertebrates, at 

 some stage in its life, viz. a larval stage, breathed by 

 functional gills, and that the adult ancestor never did 

 so. But our excessive caution in this instance would 

 be singularly misplaced ; as it would involve one of two 

 equally unlikely and gratuitous suppositions, either 

 that the class of Fishes has no connection at all with 

 the higher vertebrates, or else that the Fishes were 

 originally land animals with a fish-like larval stage, 

 which have retrograded in development and lost all 

 the higher terrestrial phases of their organization. 

 Needless to say there is no shadow of evidence for 

 either of these suppositions, and though morpho- 

 logical science may not carry with it the conviction of 

 exactitude, it is not necessary on that account to 

 make it palpably absurd. 



