378 THE BIOLOGY OF BIRDS 



to the transmission of individually acquired modifications. 

 To this it may be answered that callosities sometimes appear 

 as a species-character in places where they are not known to 

 be of any use, and that these callosities must be regarded, 

 like many other integumentary peculiarities, as bodily 

 expressions of a germinal variation. If this be granted, it 

 may be allowed that callosities in appropriate places may 

 have arisen from germinal variations which happened to be 

 useful or adaptive. In other words, they may represent 

 outcrops from within rather than indents from without. It 

 is also possible that a callosity may be the necessary growth- 

 correlate of some much bigger variation in conditions of 

 skin-pressure, blood-vessel distribution, and innervation. 



A concrete case of interest is to be found in the two-toed 

 ostrich (Duerden, 1919). When the ostrich is crouching, 

 it rests on the tips of its partly bent toes and on the upper 

 (ankle) end of its instep (tarso-metatarsus). Now there are 

 callosities on the toes and on the ankle, which are doubtless 

 parts of the inheritance, for they occur on chicks before 

 hatching. It might be concluded that these callosities had 

 become part of the inheritance through the transmission of 

 the results of pressure on the skin during the individual 

 lifetime. But things are not so easy. 



It appears that the median ankle callosity is not used — 

 nowadays at least — though it continues to be part of the 

 inheritance. If it be urged that it may have been of use 

 once, a further difficulty comes into view. Besides the 

 median ankle callosity, there is an accessory ankle callosity, 

 which begins to form after hatching and gradually becomes 

 larger and coarser. This accessory pad seems to be of much 

 use, but it is not known to be transmissible. Professor 

 Duerden ingeniously suggests that the ostrich has ceased to 

 be plastic. " In many respects the ostrich appears to have 

 reached senility [in a racial sense], and it may be that 

 structural changes resulting from external stimuli are now 

 more likely to remain transient, instead of becoming 

 impressed permanently upon the organism. This may 

 assist in some measure in understanding why the later 



