186 THE STORY OF BIRD-LIFE. 



provided with a deep keel for the attachment 

 of powerful muscles to move the wings. From 

 the general form of the body and legs there is 

 no doubt that it was a dweller on the sea- 

 shore, and could swim, and perhaps even dive, 

 easily. ^ In this, you will note, we have evidence 

 first of progress in development, in time ; and 

 secondly, of the modification of the skeleton to 

 suit changed requirements. The first bird was 

 a dweller in trees, the second a frequenter of the 

 seashore. 



In Hesperornis we see this adaptation to an 

 aquatic mode of life pushed to its extremest 

 limit. From being a dweller on the seashore 

 this bird has become a dweller on the sea 

 itself. 



Possessing abundant food in the immediate 

 vicinity of a gently sloping coast-line free from 

 violent surf, which served as a breeding-ground, 

 and with the absence of enemies, flight became 

 no longer necessary to existence, and was ex- 

 changed for great diving powers. The saving 

 effected in the non-maintenance of the wing as 

 an efficient organ of flight was applied to the 

 increased development of the leg, upon which 

 the powers of diving depended. To this end, 

 other changes were also effected. Thus, the 

 decline of the wing brought about the degener- 

 ation of the breast-bone, by the complete loss 

 of its keel. The wing itself became reduced 

 to a mere vestige, only the upper arm-bone 

 remaining; whilst the pelvis, or hip-girdle, became 

 specially modified in accordance with the diving 

 habits. Exactly what changes have taken place 



