EVIDENCES OF EVOLUTION 65 



it out again through his gills within his gill-slits. As the 

 water passes along, the red-blood corpuscles in the gills take 

 out all the oxygen they need. 



When fish are drawn out of water and left on land, they die 

 because they cannot get oxygen from the water in their usual 

 way, through their gills. When we are held under water, we 

 die because we cannot get oxygen from the air in our usual 

 way, through the lungs. 



Since this is the case, zoologists 1 expected to find gill- 

 slits in embryo 2 fish, and lungs in the embryos of animals 

 that take their oxygen directly from the air. Imagine, then, 

 the surprise of these men when they found gill-slits and 

 lungs too in the embryo of all vertebrates, whether they were, 

 fish or not. 



Birds that are to live in trees and never swim, mice and 

 men, monkeys and elephants, snakes and bats and vertebrate 

 beasts of every sort — all these have gill-slits in the neck 

 during the embryo stage. At the same time those that are 

 to breathe air after birth have embryo lungs too. 



And this is not all. Even in Darwin's day the bodies of 

 animals held other mysteries which no man could explain. 

 It was well known that full-grown whales have rudiments 3 of 

 hind legs concealed under the flesh ; that all embryo whales 

 have rudimentary teeth, although after birth some kinds 

 never have any teeth in their jaws ; that the python and 

 the boa have rudiments of legs never used, never desired ; 

 that calves before they are born have beginnings of front teeth 

 in the upper jaw which never cut through the gums after 

 birth. Notice that no cow has front teeth on the upper jaw. 



1 A zoologist is a student of animal life. 



* Before birth a creature is called an embryo. 



8 A rudiment is the beginning or foundation of any part or organ. 



