310 THE AMERICAN NATURALIST [Vol.LIV 



new relationships are repeated generation after genera- 

 tion and maintained at their full vigor for the whole life- 

 time, it is conceivable that they become so impressed on 

 the organism that they gradually overcome the old weak- 

 ening relationships of parts and appear from the begin- 

 ning in place of them, in other words, the character be- 

 comes transmissible, the new ties become the heritage of 

 the organism. This, of course, is no proof of the in- 

 heritance of acquired characters, but may help us to con- 

 ceive its possibility in the light of considerations engen- 

 dered by the callosities in the ostrich. 



The skin is more likely to show responses to environ- 

 mental stimuli and to the general activities of an animal 

 than the internal organs on account of its superficial, 

 exposed position, and callous pads are among the sim- 

 plest of structural responses and their formation is read- 

 ily understood. Where temporary, as on the human 

 hand, they are by no means likely to impress themselves 

 permanently as new interrelationships on the surround- 

 ing parts. Where, however, as in the ostrich, they would 

 form from the beginning and persist throughout life, 

 from generation to generation, it is more conceivable 

 that they would impress themselves on the constitu- 

 tion of the bird and their time of appearance would un- 

 dergo acceleration with an independence of the primary 

 stimulus. 



The accessory, non-transmissible callosity at the ankle 

 has not yet impressed itself so forcibly upon the gen- 

 eral structural relationships as permanently to disturb 

 the normal tendencies, and it has to be formed anew in 

 each generation from direct stimuli. The hereditary 

 median thickening is the primary one, and may well jus- 

 tify us in thinking that the three-toed, ancestral stage of 

 the ostrich was of long geological duration ; the new pad 

 formed by the two-toed bird is more recent and has failed 

 as yet to attain transmissibility. It may be that in its 

 early days a race is more responsive to adaptive, struc- 

 tural changes than at a later period. In many respects 

 the ostrich now appears senescent, and may well be ex- 

 pected to be less plastic than in past ages. 



