THE STUDY OF STRUCTURE. 343 



sometimes incompletely formed. Other organs may 

 be similarly spoilt in the making. They illustrate 

 arrested development. (&) Some animals lose, in 

 the course of their life, some of the promiseful 

 characteristics of their larval life; thus parasitic 

 crustaceans at first free-living, and sessile sea-squirts 

 at first free-swimming, always undergo degenera- 

 tion. The retrogression can be seen in each life- 

 time. But the little Kiwi of New Zealand, with 

 mere apologies for wings, and many cave fishes and 

 cave crustaceans with slight hints of eyes, illustrate 

 degeneration which has taken such a hold of the 

 animals that the young stages also are degenerate. 

 The retrogression cannot be seen in each lifetime, 

 evident as it is when we compare these degenerate 

 forms with their ancestral ideal. (c) But among 

 " rudimentary organs " we also include structures 

 somewhat different, e.g., the gill clefts which persist 

 in embryonic reptiles, birds, and mammals, though 

 they serve no obvious purpose, or the embryonic 

 teeth of whalebone whales. These are " vestigial 

 structures," traces of ancestral history and intel- 

 ligible on no other theory. The gill clefts are used 

 for respiration in all vertebrates below reptiles; the 

 ancestors of whalebone whales doubtless had func- 

 tional teeth. In regard to these persistent vestigial 

 structures, it must also be recognised that we are not 

 warranted in calling them useless. Though they 

 themselves are not functional, they may sometimes 

 be, as Kleinenberg suggests, necessary for the growth 

 of other structures which are useful. 



The foundations of comparative anatomy were 

 laid by Cuvier. But the historical lineage shows the 

 influence of another strain, that of the evolutionary 

 anatomists, like Goethe and Etienne Geoffroy St.- 



w 



