THE CHANGING WORLD OF LIFE 



553 



and four in the birds and mammals. The embryos of the entire group 

 first develop the two-chambered heart; if the adult has more, these two 

 are properly divided during embryonic development and the adult has 

 the correct number. 



Such cases are so numerous and the implications so obvious that one 

 who carefully studies embryology can hardly fail to see relationships 

 between many different forms of life. 



Evidence from Comparative Anatomy. A comparative study of the 

 anatomy of different animals reveals structures whose presence can 

 hardly be explained by anything besides evolutionary relationships. 



Photos by Winchester 



Fig. 34.10. Vestigial legs of the python. Although most snakes show no sign of legs, 

 there are some like the python that have vestiges of them. The photo on the left 

 shows the vestigial legs on either side of the cloaca. On the right, the skeleton of the 

 same region of the body shows that there is also a vestigial pelvic girdle to which the 



legs are attached. 



The huge python snakes of Africa have vestigial legs projecting from 

 their bodies on either side of the cloaca, and other snakes have internal 

 vestiges of legs. The horse stands on the tip of one toe, yet rudiments 

 of other toes may be found higher up on the leg. Birds have a third 

 eyelid that moves across the eye from the anterior to close it ; the mam- 

 mals have this eyelid in a vestigial form, placed in next to the nose, but 

 it is not large enough to close the eye. The appendix of man seems to 

 be a vestige of an extension of the large intestine which is much larger 

 and has an important digestive function in some animals. Some gifted 

 persons can wiggle their ears with the small ear muscles, but they hear 

 no better than the rest of us. On the other hand, these muscles are of 

 importance in dogs, donkeys, rabbits, and other animals that turn their 



