DESCENT WITH CHANGE 



357 



Man 



Bird 



the appendages of the Crayfish are not only serially homologous 

 among themselves, but are also homologous with the appendages 

 of all the other members of the class Crus- 

 tacea — just as the limbs of one Vertebrate 

 are homologous with those of all other 

 Vertebrates. (Figs. 64, 65, 227.) 



Another class of facts presented by 

 comparative anatomy is derived from 

 the so-called vestigial organs. In Man 

 there are nearly a hundred structures 

 which apparently are useless and some- 

 times are harmful. One thinks at once 

 of the vermiform appendix of the large 

 intestine, apparently a remnant of an or- 

 gan that serves a useful purpose in certain Fig. 229. — The nicti- 

 vegetable-feeding (herbivorous) Mammals. tatin s membrane, or third 

 But equally suggestive are the muscles yei 

 of the ear, which in some individuals are sufficiently developed 

 to move the external ear; or the so-called third eyelid at the in- 

 ner angle of the eye which corresponds to the lid (nictitating 



membrane) that moves laterally across 

 the eye of Reptiles and Birds; or the 

 terminal vertebrae (coccyx) of the human 

 spinal column. Other animals likewise 

 possess many such structures. Porpoises 

 have vestiges of hind limbs enclosed 

 within the body, and certain species of 

 Snakes bear tiny useless hind legs. The 

 splint bones of the Horse are remnants of 

 lost toes. (Figs. 109, 206, 229, 230, 234.) 

 In another class of cases, the organs, or 

 remnants of organs of a lower form are 

 altered or completely made over, as it 

 hind limbs of a Snake, were, into new organs of the higher form. 

 Python. /, femur or thigh The milk glands of Mammals are trans- 

 bone; il, ilium or hip bone. c i *. i j * xi_ i • u-i 4.1 



(From Romanes.) formed sweat glands of the skin, while the 



poison glands of Snakes are specialized 

 salivary glands. During the embryonic life of Vertebrates there 

 are gill slits, all of which vanish in higher forms except one 

 pair which remains as passages (Eustachian tubes) connect- 



