28 THE STORY OF THE BIRDS. 
first. This is the case (even more marked) with their 
downs also. In the adult cassowaries and emeus this 
aftershaft (as it is called) is as large as the other or 
true feathers, but in their nestling downs it is smaller, 
showing quite probably that its size in their case is a 
later development. 
If it were once a general form of underwear it 
was a fashion easily changed ; for many lower, and 
most of the higher, birds seemed to have abandoned 
it. It is scarcely found at all upon the tail and wing 
quills, and in some birds it persists among the downs 
only. 
Besides thickening the plumage its purpose is not 
apparent. The only hint—perhaps a faint one—of 
its origin is that some ostrich forms (the lowest living 
birds) have double-stemmed feathers. In some moas 
(now extinct) the feather-producing sockets tend to 
pair and merge into each other on the neck (likewise 
producing double-stemmed feathers), but they are far 
apart on the rest of the body. It may be possible 
that the clustering of papille or feather-growing 
pockets has left certain places featherless ; and here 
may lie one step back in the formation of bare tracts. 
We feel sure that all birds once had after-shafted 
plumage. But what made the papzlle cluster ¢ After 
all, if we knew more of the style of a bird’s under- 
wear we might understand why it has cut its frock so. 
All of this shows how very, very young the oldest of 
our modern birds are—that is, how comparatively 
recent many of their modifications have been. 
Downs now seem developed on birds according to 
