298 



THE AMERICAN NATURALIST [Vol. LIV 



is involved in producing the "horny hand of toil"; it 

 forms in the individual in proportion to the need for it. 

 If fore-doomed to hard manual labor some advantage 

 may possibly be conceived in having the callosities in ad- 

 vance, but would be insufficient to be of any selection 

 value. 



The position resolves itself as follows : From the known 

 responsiveness of the skin of the ostrich to intermittent 

 pressure and friction and the established activities of the 

 bird it is just as certain that the sternal, pubic and alar 

 callosities could be acquired in each generation inde- 

 pendently as that similar thickenings could develop on 

 the palm of the human hand engaged in labor. If we are 

 not prepared to admit that the callosities first arose as 

 somatic adaptations and then became hereditary, we have 

 to face the alternative that at some time in the history of 

 the ostrich a change took place in its germ plasm of such 

 a nature as to give rise to a directly adaptive character, 

 altogether similar to what could be somatically acquired; 

 we have to admit that an exactly similar character could 

 be produced in two wholly different ways : (a} directly as 

 a response to the activities of the bird; (b) as a result of 

 germinal changes. The same character could be soma- 

 tically acquired and could arise germinally. 



Of course the same argument could be applied to the 

 strongly marked callosities on the toes and ankle of the 

 ostrich which are also hereditary (Fig. 5). But these are 

 not so peculiarly specific for the present purpose. Hered- 

 itary pedal thickenings occur in most animals, and even 

 Darwin 4 regarded the thickened sole of unborn infants 

 as "the inherited effects of pressure during a long series 

 of generations." The thickenings on the sternum, pubis 

 and wings are confined to the ostrich, and therefore 

 afford a more circumscribed case for discussion, hered- 

 itary transmission from any other type being placed out 

 of consideration, though it is not unlikely that some of 

 the other Batites may have corresponding structures. 



An acquired, non-transmissible, callous pad, presum- 

 ably due to a change in the crouching habit of the ostrich 



