EVOLUTION 



349 



the results of evolution. The likenesses in homologous structures are 

 the features not yet changed in diverse directions by evolution. In gen- 

 eral very similar animals must be closely related; that is, their common 

 ancestry dates from a comparatively recent time, so that there has been 

 time for but a small amount of divergence by evolution. Very dissimilar 

 animals which, however, have fundamental likenesses, must have di- 

 verged for a long time, that is, their common ancestors were very ancient. 



Fig. 240. — Membranous labyrinths of inner ear of various vertebrates. Each consists 

 of a saccular portion from which three semicircular canals arise. These structures are 

 homologous with one another, indicating relationship of the forms possessing them. A, of 

 a fish; B, of a frog; C, of a reptile; D, of a bird. {Modified from Retzius.) 



Comparative Embryology. — In similar manner, likenesses between 

 the embryos of different animals are plausibly explained as due to common 

 ancestry. Even in animals in which the adults are quite different, the 

 embryos are sometimes similar. This is more often true in the case of 

 parasitic animals, since adult parasites are frequently very degenerate. 

 An excellent example of a case in which comparative embryology reveals 

 common descent which the anatomy of the adult would not show is a para- 

 site, Sacculina, found attached to the under side of the abdomen of com- 

 mon crabs (Fig. 241). Sacculina, in the adult stage, is a rounded pulpy 

 mass with practically no definite structure, except a host of root-Hke 

 processes which extend throughout the crab's body and absorb the body 



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