64 Evolution and Adaptation 
also absorbed and the gill-clefts close. Lungs then develop 
which become the permanent organs of respiration. 
There are two points to be noticed in this connection. 
First, the external gills, which are the first to develop, do not 
seem to correspond to any permanent adult stage of a lower 
group. Second, the transition from the tadpole to the frog 
can only be used by way of analogy of what is supposed 
to have taken place ancestrally in the reptiles, birds, and 
mammals, since no one will maintain that the frogs represent 
a group transitional between the amphibians and the higher 
forms. However, since the salamanders also have gills and 
gill-slits in the young stages, and lose them when they leave 
the water to become adult land forms, this group will better 
serve to illustrate how the gill-system has been lost in the 
higher forms. Not that in this case either, need we suppose 
that the forms living to-day represent ancestral, transitional 
forms, but only that they indicate how such a remarkable 
change from a gill-breathing form, living in the water, 
might become transformed into a lung-breathing land form. 
Such a change is supposed to have taken place when the 
ancestors of the reptiles and the mammals left the water 
to take up their abode on the land. 
The point to which I wish to draw especial attention in 
this connection is that in the higher forms the gill-slits ap- 
pear at a very early stage; in fact, as early in the mammal 
as in the salamander or the fish, so that if we suppose 
their appearance in the mammal is a repetition of the 
adult amphibian stage, then, since this stage appears as early 
in the development of the mammal as in the amphibians 
themselves, the conclusion is somewhat paradoxical. 
The history of the notochord in the vertebrate series gives 
an interesting parallel. In amphioxus it is a tough and firm 
cord that extends from end to end of the body. On each 
side of it lie the plates of muscles. It appears ata very 
early stage of development as a fold of the upper wall of 
