56 THE STRUCTURE AND LIFE OF BIRDS chap. 



hitherto had passed muster in a mediocre world, ruined 

 by the splendid acquirement of flight, unaccompanied 

 by the other variations without which it would be 

 indubitably fatal. What if wings had been fully 

 developed so that the fore limbs could no longer be 

 used in walking, while, as yet, the hind limbs 

 had not grown strong so as to make the quadruped 

 a biped ? What if the long legs, so necessary to 

 many wading birds, had not been matched by the 

 length of the neck ? How would such a Tantalus on 

 stilts have reached to the ground to get his food ? 

 What would the large expanse of wings have availed, 

 if the muscles to work the wings had not been de- 

 veloped in a corresponding degree ? How would even 

 the fully developed muscles have been equal to a strain 

 so hard, and often so prolonged, had not the heart 

 been so improved that the arterial and venous blood, 

 the fresh and the exhausted, could be kept separate ? 

 And what would have been the use of a first-rate heart 

 without first-rate lungs to aerate the blood that was 

 to feed the whole body ? And without an excellent 

 digestive apparatus how could any other part of the 

 system be vigorous ? And without the power of 

 sitting firmly on an elevated perch when asleep, the 

 newly-gained power of flight, the dismay of all enemies 

 during the clay, might only have put off the hour of 

 capture and destruction till the night. This is one of 

 the greatest difficulties which the theory of evolution 

 presents, but it is not insurmountable. To begin with, 

 variations are almost always small. We must not 

 imagine the sudden development of a perfect wing. 

 Moreover, slight variations are perpetually occurring ; 



