84 THE BIOLOGY OF BIRDS 



(i) The Wing of a Bird and the Arm of a Man — 

 Homologous but not Analogous. 



(2) The Wing of a Bird and the Wing of a Butterfly — 

 Analogous but not Homologous. 



(3) The Wing of a Bird and the Wing of a Bat — 

 Homologous and Analogous. 



The transition from water to dry land, whether prompted 

 by necessity or the spirit of adventure, or by both, made 

 for progress, but it was attended in the first instance by 

 obvious disabiHties. 



(i) The supply of oxygen was more copious but it was 

 more difficult to tap, especially when the skin of the immi- 

 grants on to the land had to be protectively hardened. 

 Every one knows that the solution arrived at was having 

 internal surfaces — notably the lungs of Amphibians, 

 Reptiles, Birds, and Mammals. The branching air-tubes 

 of insects, taking air to every hole and corner of the body, 

 illustrate another idea. 



(2) In water there is great freedom of movement, up 

 and down, to right and left, forward or backward, in many 

 planes. But the transition to dry land implied restriction 

 to one plane — the surface of the earth. This involved 

 great risks and led no doubt to improved musculature and 

 to better brains for controlling the necessarily precise 

 movements. We can understand why many terrestrial 

 animals have added to their possibilities of movement by 

 becoming burrowers on the one hand and arboreal on the 

 other. But flight is a fine illustration of the way in which 

 a disability leads to a new advance. The cat's discomfiture 

 as the stalked sparrow flicks into the air is a familiar 

 illustration of the survival value of flight. 



(3) Another disability attendant on the possession of 

 the dry land concerns the eggs and the young. Aquatic 

 animals can shed their eggs or liberate their young with 

 much more security than is possible for terrestrial animals. 

 For the water is present as a universal cradle, supporting 

 and washing the delicate young lives. But to lay eggs 

 or to liberate young ones on the dry ground is often to 



