FLIGHTLESS BIRDS. 29 
obtaining food or for the avoidance of enemies, and as 
such, under the usual conditions of life, it has given its 
possessors an enormous advantage in the struggle for 
existence ; but in the comparatively rare cases, where the 
above conditions of existence are satisfied without calling 
in the aid of flight, then we find this function dispensed 
with, as involving an expenditure of nerve force no longer 
required. 
We need not, however, go to New Zealand for an 
example of a flightless bird. The domestic Duck of our 
farm yards furnishes us with an instance in which similar 
causes have produced similar results, though, of course, 
the favouring conditions, such as the protection from 
enemies, have here been artificially produced. The excess of 
food and the consequent increase of body weight, associated 
with an absence of any need for the use of the wings, 
have produced the saine results, as we see in the birds 
just described, though doubtless, in the case of the Duck, 
a certain amount of artificial selection of the larger and 
heavier birds for breeding from, as being the more valu- 
able, has operated in favour of increased weight of body. 
We do not, indeed, see in the domestic Duck the skeletal 
changes observed in Stringops, for instance, but this 1s 
probably mainly due to the fact that the factors have been 
in operation for a much shorter period of time, for, as a 
matter of fact, a tendency in the same direction can be 
traced. 
Darwin, as is well known, found that the bones of the 
wing of the domestic Duck weigh less and the bones of 
the leg more, in proportion to the whole skeleton, than do 
the same bones in the wild Duck; but so far as I know 
the condition of the carina sterni in this bird has not 
received attention. 
It occurred to me, however, that the flightless condition 
