ADAPTIVENESS AND PURPOSIVENESS 323 



sinking in. The African egg-eating snake, Dasypeltis, has 

 very few teeth and it would not be profitable to crack the 

 eggs in its mouth ; the egg slips intact into the gullet, where 

 it is met by the sharp points of the inferior spines of a 

 number of vertebra). These project into the gullet and cut 

 the egg-shells, so that none of the precious food is wasted. 

 The spines are said to be actually tipped with enamel, the 

 hardest of all tissues. The empty broken egg-shells are 

 always returned. 



An adaptation that gives us pause is the 'egg-tooth' 

 found at the tip of the bill in many young birds, and used 

 by them to break a way through the imprisoning egg-shell. 

 It is a hard thickening of horn and lime at the tip of the 

 bill, and since it develops before the horny ensheathment 

 of the beak it may be a residue of a very ancient scaly 

 armature in Reptilian ancestors of birds. Be this as it 

 may, the instrument is an effective one and it is used only 

 once! What happens is this: the young bird ready to be 

 hatched thrusts its beak into the air-chamber that forms 

 at the broad end of the egg; air rushes down the nostrils 

 and fills the lungs for the first time; in the exhilaration 

 of this first breath the unhatched bird knocks vigorously 

 at the shell and breaks open the prison doors. After a few 

 days, in most cases, the egg-tooth, having done its work, falls 

 off, a well-adapted instrument that functions only once. 



But there is a further detail which is of much interest. 

 The bill and its egg-tooth are only the instruments; what 

 about the musculature which works these? Prof. Franz 

 Keibel has inquired into this in the case of the unhatched 

 chick and duckling. He finds that the work is done by a 

 muscle called the musculus complexus, and that this is very 

 markedly hypertrophied for some time before hatching. On 



