NATUBAL SELECTION 



673 



others (Romer, 1933, pp. 308-314; see also 

 Fig. 246). 



In some cases a character may become 

 harmful to an organism, and selection pres- 

 sure will act to reduce it (pp. 637, 677). 

 Simpson (1944, p. 88) cites instances 

 among closely related mollusks in which 

 some lose their shells because the shell im- 

 pedes locomotion and requires much food 



and in others vestigial structures may be 

 identified and homologized with the func- 

 tional organs of more primitive animals, 

 both living and fossil. 



Probably the best studied examples of 

 convergent regression are found in the cave 

 habitat, in which pigment (Pavan, 1946; 

 Rasquin, 1947) and photoreceptive organs 

 have degenerated among various animals. 



eust'dchian hbe 



Fig. 246. The evolution of the vertebrate ear. A, Cross section of a fish skull through the 

 internal ear, consisting of sacs and semicircular canals only. B, Cross section of an amphibian 

 skull, showing the modification of the fish hyomandibular bone to form the stapes. C, Cross 

 section of a mammal-hke reptile skull. D, Cross section of the skull of man, showing the mal- 

 leus and incus modified from skull bones that formed the jaw joint in the lower forms. E, A 

 primitive land animal and a mammal-like reptile, showing the relation of the eardrum to the 

 jaw joint, a, articular bone; d, dentary bone; eu, eustachian tube; hm, hyomandibular bone; 

 i, incus; m, malleus; me, cavity of middle ear; oe, cavity of outer ear; q, quadrate bone; s, 

 staples; sp, spiracle; tm, tympanic membrane. ( From Romer. ) 



for its development, while others develop 

 stronger shells because of their value as a 

 protection from predators and from environ- 

 mental dangers. Chiton with a strong well- 

 developed shell and Neomenia without a 

 shell seem to have evolved fairly recently 

 from a common ancestor with a moder- 

 ately developed shell. 



In other cases, as in the eyes of cave 

 animals, the organ does not take on another 

 function, nor is its presence haiTnful, but 

 it seems merely to lose its value to the or- 

 ganism. No vestige is left in some species. 



Eyes are reduced in cave species of sala- 

 manders, fishes, beetles, millipedes, cray- 

 fish, isopods, amphipods, harvestmen, and 

 spiders. Even the eye spots of cave flat- 

 worms are absent or reduced. Convergent 

 regression is also apparent within many of 

 these groups (Hubbs, 1938; Van Name, 

 1936, p. 465). 



The absence of a character may be ow- 

 ing to its lack of development in evolution- 

 ary history (genetic absence), or to regres- 

 sive evolution (probably partial genetic 

 presence). The regression of a character 



