PROFIT AND LOSS IN THE BIRDS. 195 
upon their legs, or rather the scales have all merged 
into one sheath. The shanks of the melodious thrushes 
are, for part of the way, entirely scaleless (see cut, 
page 161). We can not see how this can in any way 
be a compensation ; so that the best we can do is to 
cloak our ignorance by 
calling it a correlation. Ny ae) ee 
Since the bird left the | 079 4a | 
reptilian state by losing oa | 
scales partly, this may be 
regarded by some as 
merely the result of a 
scale-losing momentum ; 
but why it should be co- 
ordinated with, or ac- 
companied by more than 
three pairs of song mus- 
cles we can not say, un- 
less the acquisition of 
song muscles is also the 
highest development 
away from the lizards. 
The loss of one or two 
primary wing quills seems 
to run quite unexplain- 
ably almost parallel with these last two changes also. 
Lastly, besides the loss of teeth, there have been 
great changes in the skull, in keeping with progress 
or degeneration, especially in the bones of the palate 
or skull floor. These seem to be related to the bird’s 
brain, or its intelligence, and to its habits. They are 

Melodious thrushes. 
