STUDIES or THE MACKOCHIEES. 
389 
the original Passerine stock, have long since deviated from the 
latter. They are true Passeres considerably modified, which 
modifications in several instances may be traced to the adoption of 
new habits, and are really physiological adaptations of structure. 
For instance, through ages of time they no doubt have gradually 
attained their increase of size in the gape, which enables them 
to take insect prey upon the wing with greater certainty and 
ease, and at the present time constitutes one of their best 
distiuctional characters. 
Whatever may be the physical principle involved that seems 
to demand a brevity of the brachium to suit their flight, we cer- 
tainly can now perceive that a proportional shortening of the 
humerus is going on. Their flight, however, is not of such a 
vigorous nature as yet to demand much increase in the size of 
their pectoral muscles, and the consequent deepening of the Carina 
of the sternum for their due attachment, nor the unnotched body 
of that bone to afford a more stable surface for the origin of 
those muscles. But with the present configuration of the coun- 
tries they inhabit, and where their food is now to be had in 
abundance, no doubt they will long retain their present habits, 
and consequently their present structural organization. 
Suppose, however, at some time in the world’s history, ages ago, 
there were certain large areas inhabited by the original Hirundine 
stock, sufficiently differentiated from the existing Passerine types, 
in which from some cause there was a diminished supply of 
natural food — the insects which they had been accustomed to take 
on the wing. This would at once seem to demand in the organi- 
zation of the Swallows an increased rapidity of flight, in order 
to secure for themselves and their young sufficient food during 
the course of the day. It would also lead, perhaps, to an increase 
in the size of the mouth, that this food might be captured with 
greater certainty. Further, they would probably be obliged to 
remain longer upon the wing. Continued for a sufficient length 
of time, such causes would be sure to work structural changes 
in the economy of these birds, and modifications would in con- 
sequence follow in their icing-structure, in the size and strength 
of their pectoral muscles, with an increase of the bony surface 
of the sternum, botli in body and carina, from which these latter 
muscles arise ; and finally, among certain other minor changes, 
we might find in consequence of the last-named requirement a 
suppression of parts in th e. feet and certain muscles of th e pelvic 
limb , as the owners would now rarely perch or walk. 
