180 The Bird 
seem to us. In changing, Nature seems to have tried 
numberless experiments, only a few of which have sur- 
vived. For example, we know that fish breathe by a 
sort of swallowing, the water being taken in at the mouth 
and poured out through the gill-clefts. So in frogs and 
salamanders we find that, although they possess lungs, 
yet they still employ a swallowing process to get the air 
down their throats. This is the reason why a frog will 
suffocate if its mouth is held open. There are certain 
salamanders which are wholly without lungs, their moist 
skin being so vascular that the blood is purified through 
it. But strange to say, these amphibians still swallow and 
swallow, as did their ancestors, although no air passes 
down their throats, and indeed there is no place to which 
it could go! As we have seen elsewhere, birds exhale 
air largely by the action of certain abdominal muscles. 
Watch a goldfish rise to the top of the water and eject or 
gulp down a bubble of air, and observe the rapid breath- 
ing of a bird, and you have the two extremes before you 
—the swim-bladder of ages ago and the wonderful lungs 
of a bird of to-day. 
The Heart and the Life-blood 
Perhaps the most wonderful organ in a bird’s body 
is its heart. In the very lowest of back-boned animals 
the heart is merely a long tube, in fact a simple artery 
or vein, which contracts at certain intervals and so pro- 
pels in a forward direction the fluid which it contains. 
A fish may almost be said to have its heart in its head, 
so far forward in its body is it placed; nevertheless, as 
