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 at a very 

 early stage of development as a fold of the upper wall of 



