258 THE STRUCTURE AND LIFE OF BIRDS cHaP. 
certain point, its lower face looking backward, the effect 
will be the same, without the help of the Latissimus. 
During the upstroke the hind-quarters drop no more 
than the rest of the body. The centre of gravity 
is always behind the shoulder; in fact, when. the 
hinder part of the body is raised, it retreats still further. 
It ought not to fall in a vertical line between the points 
of the wings’ attachments any more than a skater’s 
centre of gravity, when he describes a curve, ought to 
be directly over his skate. The action of the wings, 
nicely regulated by the Latissimus, is to raise the 
hinder and lower the anterior part of the body, and, if 
the bird were balanced horizontally between them, 
would send him heels over head. The fact that the 
main weight is behind prevents this, and enables him 
to choose his course. 
If an animal uses for progression at one time his 
fore limbs only, at another his hind limbs, the centre of 
gravity question cannot fail to be interesting, as equi- 
librium must be maintained under two different sets of 
circumstances. How well the point is situated for 
flight, I have shown. In standing or walking, the 
thigh-joint is a long way behind the main bulk of 
the body, and the long toes are necessary to give 
support at the required point. The horizontal or 
nearly horizontal carriage of the body calls into play 
muscles which, passing from the posterior end of 
the pelvis to the thigh-bone, pull the hind-quarters 
downward, and so raise the head and shoulders. The 
more upright the bird, the farther back the centre of 
gravity, and the less the pressure upon the toes. 
When standing, a bird is in the position of a man who 
