362 BIOLOGY AND ITS MAKERS 
arterics 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 
Fic. 110.—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. 
