314 A HISTORY OF BIRDS 



tion of the skeletal parts can be seen to-day in the Game- 

 birds {Galliforines). Herein the keel of the sternum is much 

 hollowed out in front, and especially so in the Turkeys, where- 

 in the hypocleideum of the furcula is also of great length. 

 Now in the Game-birds the crop is also of great size, and is 

 lodged in a fashion resembling that which obtains in the Hoat- 

 zin. That is to say, it is supported, when the bird is at rest, be- 

 tween the arms of the furcula, and in part by the breast-muscles 

 on either side thereof As flight became less imperative, and 

 the dwindling of the sternal keel more marked, more space 

 would be at the disposal of the developing crop allowing a 

 greater increase in size, in proportion as it took over the duties 

 of the gizzard. If in the Turkey the size of the crop increased 

 as the powers of flight decreased, we should have a modification 

 of the skeleton precisely similar to that of the Hoatzin. 



The Hoatzin, as is well known, is a bird whose powers 

 of flight are extremely limited. It is believed, indeed, never 

 to come to the ground, but to pass its whole life among trees 

 overhanging water, and feeding upon the fruit of an astringent 

 aroid. This peculiar food may have been possible, only because 

 the ancestral Hoatzin had a crop which was unaffected thereby, 

 possibly because thicker-walled and gland-secreting, whereby 

 the process of digestion began before the food was passed on 

 to the stomach. This being so the action of selection would 

 be to increase the size of the crop and to decrease the size of 

 the gizzard. 



Hunter succeeded in feeding a Gull {Larus tridactylus) for 

 a year on grain, at the end of which time the lining of the 

 gizzard had acquired the horny surface characteristic of grain- 

 eaters, while another experimenter, Holmgren, showed that 

 the gizzard of Pigeons fed on meat lost this horny consistence, 

 and assumed the soft character of that of a bird of prey. But 

 no one would expect to find the offspring of such Pigeons 

 similarly affected as a consequence of the abnormal diet of 

 their parents. In the days of cock-fighting the combs of Game- 

 cocks were invariably pared down, but this did not affect their 

 male offspring. Yet it has been urged that the long tails of 

 the Japanese " Long-tailed Fowls," which may attain a length 

 of as much as seventeen feet, have been developed by the 

 accumulated effects of long stimulus applied by gently and 



