STUDIES OE THE MACROCHIRES. 



389 



the original Passerine stock, have loug 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 

 distinctional characters. 



Whatever may be the physical principle involved that seems 

 to demand a brevity of the brachiuin 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 tliat bone to afford a more stable surface for the origin of 

 those muscles. But witli 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, i)crhaps, 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 tiuie, such causes would be sure to work structural changes 

 in the economy of these bii-ds, and modifications would in con- 

 sequence follow in their tvincj-stracture, in the size and strength 

 of their pectoral muscles, with an increase of the bony surface 

 of the sternum, both in body aud carina, from which these latter 

 muscles arit>e ; and finally, among certain other minor changes, 

 we might find in consequence of the last-named requirement a 

 suppression of parts in feet and certain muscles of pelvic 

 Unibj as the owners would now rarely perch or walk. 



