INHERITANCE IN THE <>-T/;l< // 



293 



mine how far the skin would respond to unusual friction 

 and pressure. With its legs widely apart, the chick 

 naturally lay almost prone upon the ground, the inner 

 side of the ankle constituting- a feeble support, the tarso- 

 metatarsus having here a projecting knob. The chick 

 was able to raise itself slightly upon the latter and also 

 to drag itself along the ground. It was kept alive for 

 about ten days, and in that time developed a very con- 

 spicuous callous thickening over the inside of the meta- 

 tarsal knob just below the ankle, the normal hereditary 

 callosity along the back of the ankle being unused. The 

 thickening was covered with the minute scales present 

 over the leg generally, but the degree of friction was too 

 intense and continuous for the skin wholly to adapt itself, 

 and a slight abrasion occurred at the apex of the thick- 

 ening, as in the human hand where pressure and friction 

 are applied too continuously for the callous formation to 

 keep pace with them. 



The sternal and pubic callosities are not the only ones 

 in the ostrich which appear to represent adaptive re- 

 sponses to the special habits of the bird. When taking 

 its frequent sand-baths, it rolls about in the dry sand or 

 dust, from side to side, and at the same time uses its 

 wings in an oar-like manner. During the process the 

 under surface of the latter is dragged over the ground 

 and then turned upwards, inwards and backwards, scat- 

 tering the sand or dust over the body generally, first 

 from one wing and then from the other. The front or 

 pre-axial border of the wing is necessarily subjected to 

 much friction, and develops slight callous areas wherever 

 the internal bones project. Further, the third digit of the 

 wing, which is usually buried in the flesh, is occasionally 

 found projecting freely from the under surface, and its 

 tip naturallv comes in for a good deal of rough wear as 

 the latter is dragged along the ground. In response, it 

 becomes knob-like and thickened, the surface showing the 

 characteristic callous markings (Fig. 3). The free tip 

 of the supporting phalanx is also knobbed. 



Taking into account the responsive nature of the skin 



