364 



BIOLOGY AND ITS MAKERS 



arteries of the head and the neck undergo great changes long 

 before the chick is hatched. Similar gill-clefts and similar 

 arrangements of blood-vessels appear also very early in the 

 development of the young rabbit, and in the development 

 of all higher life. Except for the theory of descent, such 

 things would remain a lasting enigma. The universal pres- 

 ence of gill-clefts is not to be looked on as a haphazard 



FIG. no. The Jaws of an Embryonic Whale, Showing Rudimentary 



Teeth. 



occurrence. They must have some meaning, and the best 

 suggestion so far offered is that they are survivals inherited 

 from remote ancestors. The higher animals have sprung 

 from simpler ones, and the gill- slits, along with other rudi- 

 mentary organs, have been retained in their history. It is 

 not necessary to assume that they are inherited from adult 

 ancestors; they are, more likely, embryonic structures still 

 retained in the developmental history of higher animals. 



