DISTANT RELATIONS. 101 



limbs and higher sense-organs so completely 

 atrophied that only his earlier history allows 

 us to recognise him as a vertebrate by descent 

 at all. He is in fact a representative of 

 retrogressive development. The tadpole, on 

 the contrary, goes on swimming about freely, 

 and keeping the use of its eyes, till at last a 

 pair of hind legs and then a pair of fore legs 

 begin to bud out from its side, and its tail fades 

 away, and its gills disappear, and air-breathing 

 lungs take their place, and it boldly hops on 

 shore a fully evolved tailless amphibian. 



There is, however, one interesting question 

 about these two larvae which I should much 

 like to solve. The ascidian has only one eye 

 inside its useless brain, while the tadpole and 

 all other vertebrates have two from the very 

 first. Now which of us most nearly repre- 

 sents the old mud-loving vertebrate ancestor 

 in this respect ? Have two original organs 

 coalesced in the young ascidian, or has one 

 organ split up into a couple with the rest of 

 the class ? I think the latter is the true 



