THE EVIDENCE FROM DEVELOPMENT. 



189 



But reptiles, birds, and quadrupeds are lung-breathers, and possess 

 gills at no period of their life. Why, then, it may be asked, should 

 they invariably develop in their early life gill-arches and gill- 

 clefts which bear no relation to the wants of their adult existence ? 

 The gill-arches of reptiles, birds, and mammals never develop gills ; 

 and even the gills and gill-clefts of tadpoles (depicted in Fig. 105, g) 

 disappear when these animals become adult toads, frogs, and newts. 

 Why, then, it may be repeated, does this seeming irrationality and 

 useless expenditure of creative power in nature exist ? 



The true and only answer to such a pertinent query is, that the 

 gills and gill-arches of higher Vertebrates bear reference to a former 

 condition of matters. They relate to anterior stages of vertebrate 

 existence, when the ancestors of lung-breathing animals were repre- 

 sented by gill-bearing and aquatic forms. Gill-arches and gill-slits 



FIG. 105. TADPOLES OF FROG. 



thus appear as a true legacy and inheritance from an aquatic ancestry. 

 In the higher Vertebrata the first gill-opening becomes converted 

 into structures and parts connected with the ear. The remaining 

 clefts disappear, whilst the gill-arches themselves contribute to form 

 the tongue-bone (hyoid bone) and the small bones or vesicles of the 

 internal ear. Only on the theory of descent with modification can 

 we rationally explain the presence of now useless structures such as 

 the gill-arches and gill-clefts of lung-breathing Vertebrates. On this 

 principle, " we may cease marvelling," says Darwin, "at the embryo of 

 an air-breathing mammal or bird having branchial slits and arteries 

 running in loops, like those of a fish which has to breathe the 

 air dissolved in water by the aid of well-developed branchiae." 



