56 THE STRUCTURE AND LIFE OF BIRDS cuap, 
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 day, 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 ; 
