J. E. DUERDEN 169 



we are able to interpret one in terms of the other. In some instances 

 there is little or no departure from the Mendelian proportion for hetero- 

 zygotes, and in these we may deem the genetic factors to be fairly fixed 

 and stable for the time being, and at their present degenerative phase ; 

 while again we get a departure from the expected proportion and a 

 diversity in the degree of expression which can be understood only on 

 the assumption that the factors are in a variable phase. 



In practically all cases the departures from the normal are retro- 

 gressive, indicating that the germinal changes are degenerative in their 

 tendency. Where, as in Table VI, the reverse is the case it may be 

 interpreted as revealing that the germ factors are still present, but in 

 such a weakened state that expression is gained only under peculiarly 

 favourable circumstances ; for the many results, in Table V show that 

 where the claw is somatically lost expression is not likely to be regained. 

 Both reversions and additional losses may be expected to occur as 

 irregularities when the genetic factors are reduced to near the limit 

 of expression. 



Entirely independent of any extrinsic influence as the claw is found 

 to be, the various sizes in which it expresses itself — strong, medium, 

 weak, integumental — can only be regarded as proof that the factorial 

 degradation is also o£ a slow, continuous nature ; and it is reasonable to 

 suppose that where expression altogether fails the genetic factors may 

 yet be present in the germ plasm in various degenerative phases, though 

 too weak to gain somatic manifestation. As in the case of plumage 

 degeneration the appearance of the claw on one side and not on the 

 other, or even a marked difference of expression, is also deemed to be 

 evidence of fluctuating weakness on the part of the factors concerned. 



SCUTELLATION OF MiDDLE TOE. 



Along the front of the tarsus of the ostrich extends a series of large, 

 nearly rectangular epidermal scales which in most cases pass uninter- 

 ruptedly to the end of the big middle toe (Fig. 7). Over the leg the 

 scales remain approximately of the same size, but at the junction with 

 the toe they often become narrower, enlarging again distally. In flying 

 birds generally it is found that the scutellation divides at the tarso-pedal 

 joint, and is continuous with the smaller scales over the upper surface 

 of the four toes. In the ostrich however, as noted in the previous 

 section, a large interval invariably occurs between the tarsal scales and 

 those on the small toe, the latter only numbering seven to ten and 



