Ill RELATIONSHIP TO REPTILES 31 
The organ of respiration it then uses is in the tail, 
and cannot, of course, be a gill, though it is doing the 
work of one. In one and the same animal we some- 
times have analogous organs. For instance, a cater- 
pillar has only three pairs of legs properly so called. 
The hind “legs” before mentioned are only growths 
of the skin, and do not survive beyond the caterpillar 
stage. On the other hand the tails of all vertebrates 
are homologous, however different the purposes for 
which they are used: the new-world monkey’s for 
climbing, the porpoise’s for swimming, the kangaroo’s 
as a leg, the giraffe’s and many others as fly-flappers, 
the bird’s for guiding his flight. Many fish have a 
swim-bladder, which is filled with air and gives them 
buoyancy. This organ was thought by Darwin to be 
the same organ as the lungs of mammals. If so, it 
would have been a paragon example of homology. 
Unfortunately, it is an outgrowth from the back of 
the alimentary canal, whereas the opening to the 
lungs is from the front. By an extension two organs 
in the same animal are said to be homologous; for 
instance, the Humerus is homologous to the Femur, 
the corresponding bone in the hind limb. To prove 
relationship we must look for true homologies, as 
mere analogies prove nothing. 
_ Here are some of the most striking features common 
to birds and reptiles. 
(1) A single condyle or rounded projection in the 
skull fits into a cup-like hollow in the centrum or 
thickened base of the first or Atlas vertebra, which 
is so short as to be hardly more than a ring of bone. 
At one point in the rim of the cup there is a notch, and 
