Birds are usually reg-arded by evolutionists as having^ des- 
cended from reptile-like ancestors but it should be carefully 
noted that this docs not mean that they are in any way derived 
from the familiar present day lizards and snakes, etc., which are 
immediately called to mind by the use of the word reptile. 
Existini^f birds' and reptiles have so many Important structural 
characters in common that it seems that they may well have 
had a common ancestor and indeed Huxiey, one of the greatest 
of anatomists, was so impressed with the resemblance between 
the two groups that he united them into one great group which 
he called the Sauropsida. 
Without going into too much technical detail a bird may 
be satisfactorily defined as a warm blooded, feathered biped 
that reproduces by means of eggs and has the forelinnbs or 
arms modified into wings or organs of flight, but the presence 
of feathers alone is sufficient to distinguish birds from all other 
living things. Just as hairs are characteristic of the mammals 
so are feathers peculiar to birds; the hair-Hke structures seen 
about tlie base of the beak, on the eyelids and on the body of 
a fowl after it has been plucked for the table are not really 
hairs but degenerate feathers. 
With feathers therefore we can very reasonably begin our 
brief survey of the bird. Taking a broad view feathers may 
be divided into two groups, firstly there are the outer or 
contour feathers, that is the stiff, coloured feathers that meet 
the' eye when one looks at a bird and secondly there are the 
uncoloured and HiuGy "down" feathers which are exposed by 
raising the contour feathers. The *'down'* forms a warm 
underclothing and as can be expected is most abundant in 
aquatic birds. Mammals also have this '*next the skin" suit 
in the form of under-fur and man emulates the example set by 
the birds and mammals in that he disposes his clothing in two 
layers. 
I'7) 
