144 THE INFANCY OF ANIMALS 



The parrots again display striking differences in the 

 form of the beak when nestling and adult are compared. 

 In the cocatiel {Calopsittacus), for example, the beak in 

 the adult is short, and the upper jaw terminates in a 

 sharp point : in the nestling, on the other hand, the whole 

 beak is larger, relatively. The upper jaw, it will be noticed, 

 difiers absolutely in shape, the tumid area pierced by the 

 nostril is larger, and if a vertical line from the point of 

 this globular swelling be drawn it will be found to pass 

 downwards in front of the lower jaw ; in the adult such 

 a line passes through its middle. 



Further, in the nestling the upper jaw terminates in a 

 broad, scoop-like tip instead of in a point, suggesting an 

 earlier stage of development, when the food was different 

 from that now eaten. The tip of this beak, it will further 

 be noticed, bears a pointed cap : this is the " egg-tooth," 

 a small, sharp-pointed, calcareous body used as a shell- 

 breaker — that is to say, as a sort of " tin-opener," for 

 therewith the bird when about to be hatched cuts away 

 a portion of the shell the more easily to effect its escape. 



The egg-tooth is not peculiar to the parrots ; it is found 

 on the tip of the beak of all young birds, and soon after 

 hatching becomes detached. It is also, be it noted, 

 found in reptiles, and in the young echidna among the 

 mammalia. Here is an instance of a structure which is 

 not an " ancestral " character in the sense in which this 

 term is used. That is to say, it does not represent a some- 

 time adult character, but is, and always has been, an 

 accompaniment of the final embryonic stage. 



This reference to the " egg-tooth " naturally suggests 

 the question — Are any traces to be found of teeth in the 

 jaws of young birds ? They may well be sought for, since 

 ages ago many species, like Archasopteryx, Ichthyornis, 



