THE ORIGIN OF VARIATION 43 



dismissed without some discussion. A single case will make 

 the difficulty clear. Duerden (1920) has shown that the 

 sternal, alar, etc., callosities of the ostrich, which are un- 

 doubtedly related to the crouching posture of the bird, appear 

 in the embryo. The case is analogous to the thickening of 

 the soles of the feet of the human embryo attributed by Darwin 

 (190 1, p. 49) ' to the inherited effects of pressure.' As 

 Detlefsen (I.e. p. 248) points out, this would have to be ex- 

 plained on selectionist grounds by the assumption that it was 

 of advantage to have the callosities, as it were, preformed at 

 the place at which they were required in the adult. But it is 

 a large assumption that variations would arise at these spots 

 and nowhere else. 



Detlefsen (I.e. p. 248) reasonably asks ' why it is necessary 

 to have these anticipatory hereditary callosities appear in the 

 embryo before there is any demand made upon the organism 

 . . . why do they not recur in each lifetime entirely through 

 individual adaptation, as indeed it appears they can ? . . . 

 What advantage . . . would an inherited callosity . . . have 

 over an equally effective ontogenetic one ? ' We cannot see 

 what selective advantage is involved in having them formed 

 so early, unless we appeal to some principle of ' developmental 

 convenience.' Moreover, Detlefsen asks (p. 250), ' is it not 

 extremely improbable that chance variations in the germ plasm 

 would arise to determine such callosities at exactly those 

 points and nowhere else ? Why not fortuitous variations for 

 callosities elsewhere or almost anywhere on the body — which 

 should persist, for they would have little or no lethal effect in 

 the process of natural selection ? ' 



This is a particularly interesting, but at the same time a 

 very baffling case. On the one hand we have the general 

 lack of evidence to support the ' Lamarckian ' explanation ; 

 on the other the apparent absence of any advantage in having 

 the formation of the callosities pushed back in ontogeny to a 

 stage preceding the period at which they come into use. One 

 might construct a purely hypothetical explanation in terms 

 of ' developmental convenience ' to make a case for selection, 

 but it would have very little weight if it were not supported 

 by an exact and intimate study of this particular ontogeny. 



The blindness of cave-animals and deep-seaforms (cf.p. 269, 

 Chapter VII), and the atrophy of limbs in aquatic mammals are 



