FLIGHTLESS BIRDS. 3 
keel being a projection on the anterior surface of the 
sternum to which are attached the great pectoral muscles, 
the chief agents in the movements of the wings, and hence, 
being very definitely associated with the power of flight, 
is characteristic of the great majority of birds; whilst in 
birds like the Ostrich there is no keel, the sternum being 
smooth and free from projections in harmony with the 
absence of need for large pectoral muscles. Now it is a 
belief, held by many systematists, that these two divisions 
represent two fundamentally different types of birds—that 
the great contrasts which exist between these two great 
sub-classes of birds at the present time have existed from 
the earliest times, and that the common ancestor must be 
sought, not in a flying bird originally common to both, 
but in a pre-avian or reptilian form; that, in fact, the 
divergence between the two commenced in the reptilian 
and not in the avian condition. 
Others, again, see in the Ratite the primitive type of 
birds—a stage through which all Carinate birds have 
passed in the course of their development. 
To neither of these two opinions can I subscribe, as I 
agree with those who hold the view that both Carinate 
and Ratite birds are descended from a common avian 
ancestor which possessed the powers of flight; and in 
briefly reviewing the different groups of flightless birds 
already sketched out, we shall, at the same time, consider 
some of the evidence upon which this opinion rests. 
To begin with the first group into which I divided 
flightless birds, that, namely, in which compensation has 
been obtained for the loss of function of the wings by the 
ereat development of the posterior extremities, we have, 
as already indicated, an aquatic and a land section; and 
to take the former section first, we have included in it 
only a single genus, but one of great interest, the gigantic 
