36 Evolution and Adaptation 
used for flight, the breast-bone is large and serves for the 
attachment of the muscles that move the wings; outgrowths 
from the lungs extend throughout the body and even into the 
bones and serve as air sacs which make the body more buoy- 
ant. Only one aortic arch is present, the right, and the right 
ovary and oviduct are not developed. The eyes are large 
and well developed. Teeth are absent. We have here a series 
of strongly marked characteristics such as distinguish hardly 
any other class. Moreover, the organization of existing birds 
is, in its essential features, singularly uniform; the entire 
class presenting less diversity of structure than many orders 
of Fishes, Amphibians, and Reptiles.” 1 The feathers are 
the most unique features of birds, and are not found in any 
other group of the animal kingdom; moreover the plan on 
which they are formed is essentially the same throughout the 
group, yet in no two species are the feathers identical, but 
differ not only in form and proportions, but even in the char- 
acter of the barbs and hooks for holding the vane together. 
The modification of the fore-limbs for flight is another char- 
acteristic feature; yet in some birds, as the ostrich and kiwi, 
although the wing has the same general plan as in other 
birds, it is not used for flight. In the latter it is so small that 
it does not project beyond the feathers, and in some birds, as 
in the penguins, the wings are used only as organs for swim- 
ming. 
In spite of these differences we have no difficulty in 
recognizing throughout the group of birds a similarity of 
plan or structure, modified though it be in a thousand 
different ways. 
Enough has been said to illustrate what is meant by the 
similarities of organisms on which we base our system of 
classification. When we conclude from the statement that 
all vertebrates have a skull that they owe this to a common 
descent, we do not mean that a particular structure has been 
1 Parker and Haswell: “Text Book of Zoology.” 
