196 THE STORY OF THE BIRDS. 
too technical even to mention here. The relations 
between them remain more nearly constant than any 
other one feature of structure in single groups. They 
perhaps come nearer indicating the progress of the 
bird than any one set of characters. This is likely, 
because they are so near the brain; for whenever the 
body loses an old tool or acquires a new one, some 
change must be made in the brain to correspond to 
its use, and this change affects the skull and the bones 
adjoining it. 
But profit and loss among the birds have not been 
confined to structure only, but habits also, as we have 
seen, are lost and gained. It may be interesting, 
on this next-to-the-last glance at the Story of the 
Birds, to see what such habits may hint of history. 
We have already had some of the habits that are 
shaped by structure under Tools and Tasks, Chapter 
XXIII. It is the habits which, independent of 
structure, seem so freakish frequently, or rather it is 
the vestiges of such habits that we shall now notice. 
The formation of new habits is going on among the 
birds constantly, but only observation and study from 
day to day can interpret these. 
We saw early that structure of the embryo or 
young may indicate phases through which the race 
has passed, as the frog’s young are fishlike, ete. So 
likewise there may be a sort of embryology of habit. 
If we find a young bird exhibiting a peculiar habit 
not practiced when old, we are led to believe that 
some adult ancestors once practiced this regularly, or 
that some living (and usually lower) relation does so 
