40 THE STRUCTURE AND LIFE OF BIRDS cH. 
The socket of his thigh-joint is not completely 
closed with bone: in the skeleton of the crocodile as 
in that of the bird, the cup of the socket has a hole at 
the bottom, the membrane which was there during 
life having disappeared. Several of the ribs have 
uncinate processes. 
These resemblances, to which others might be added, 
donot prove that the crocodile is the ancestor of the bird. 
The heart and the gizzard were probably developed 
independently after birds and crocodiles had arisen 
from some common and more primitive stock. It 
would be unwise to say that birds are descended from 
any existing class of reptiles. But the facts justify us 
in drawing the inference that birds and reptiles are 
related; that they had common ancestors with less of 
specialised character than either of themselves, and 
that from these ancestors each class has developed in 
accordance with its own mode of life. In the same 
way Englishmen and Chinamen come from the same 
stock ; neither race is descended from the other. The 
Darwinian theory of the descent of man is that if 
you trace upward the pedigrees of men and monkeys, 
the lines will meet, not that men are descended from 
monkeys. 
BOOKS ON THE SUBJECT, 
See at the end of Chapter IT. 
Also Newton’s Dictionary of Birds, “ Fossil Birds” ; Pycraft, 
Nat. Science, November and December, 1894, “ Archzopteryx ” ; 
Owen, Philosophical Transactions, 1863, “ Archzeopteryx.” 
