292 



the 



ALIST [Vol. LIV 



surrounding parts of the skin which remain smooth. The 

 tessellation, along with the thickening below, gives it a 

 distinctive character as compared with the pads in mam- 

 mals, which are mere thickening of the skin, and whose 

 claim to be regarded as a " character" might at times be 

 disputed. Where a callosity assumes any considerable 

 thickness the underlying bone exhibits a correlated re- 

 sponse by likewise becoming thickened, as is well shown 

 on both the sternum and pubis of the ostrich. 



The skin of all vertebrates appears to have the inherent 

 power of responding to frequently repeated pressure and 

 friction by the formation of thickenings over the bony 

 projections upon which it rests. The pads are special 

 protective adaptations to meet intermittent pressure and 

 friction, upon what would otherwise be soft vulnerable 

 parts of the body. They can arise at any part of the sur- 

 face of the skin^ and may slowly disappear when the 

 causal stimuli are no longer operative. Many of them 

 are temporary responses, acquired during a part of the 

 life-time of the individual, and come under the group of 

 adaptive somatic modifications which are non-transmis- 

 sible, though others, especially those on the under-surf ace 

 of the feet, are transmissible and may therefore be re- 

 garded as germinal in their origin. Thus similar char- 

 acters, alike in structure and function, may be either in- 

 diridually acquired and non-transmissible or germinal 

 and heritable. 



The ostrich resembles man and other animals in hav- 

 ing the inherent power to produce special callosities over 

 parts of the skin not usually subjected to pressure and 

 friction, as the following observation proves. A chick 

 was hatched in the incubator with its legs widely apart, 

 in such a manner as to be incapable of supporting itself 

 upright in the normal fashion. A deformity of this 

 nature is not unusual among both ostrich and poultry 

 chicks as a result of imperfect incubation, but can gen- 

 erally be rectified by bandaging the legs and drawing 

 them nearer together for a day or two. In this instance 

 however advantage was taken of the deformity to deter- 



