THE LIFE-HISTORY OF BIRDS 263 



development climbs its own genealogical tree or recapitulates 

 the essential characters of its ancestors as it passes on to 

 acquire those which are peculiar to itself. But this record is 

 notoriously incomplete. Short cuts are often taken, and whole 

 passages are left out. In some cases these gaps appear to be 

 due to the exhaustion of the " stirp," as in the last remaining 

 vestiges of organs, where only enough of the material originally 

 set aside to form that organ now remains to briefly indicate 

 the foundations thereof, and this is later absorbed : in others it 

 may be that the apparent gaps are really so many records of 

 discontinuous variation (p. 291). 



By way of example we may take the case of the developing 

 wing. In the embryo the hand is at first short and the digits, 

 four in number, are enclosed in a web — at least vestiges of a 

 fourth digit remain — while in the wrist more elements may be 

 traced than are to be met with in the adult. Similarly, in the 

 post-embryonic state, the wing of the young chick passes 

 through a stage which was once characteristic of an ancestral 

 adult condition. In this stage there are two free proximal 

 carpals — ^radial and ulnar — ^and a separate semilunar mass of 

 bone closely approximated to the bases of the metacarpals 

 which at this stage, though closely apposed, are free. This is 

 but a transitory stage in living birds, but in the ancient Archas- 

 opteryx it persisted throughout life. In modern birds this semi- 

 lunar mass fuses with the metacarpals, while at an earlier stage 

 than that described as corresponding with the Archaeopteryx, 

 this distal row of carpals was made up of three distinct elements, 

 while a third — the intermedium — is sometimes met with in the 

 proximal row wedged in between the radius and ulna, as in 

 many reptiles. And so again with the digits. In the Ostrich 

 occasionally, and in the Penguin, each of the digits in the 

 embryo terminates in a claw, while in many birds the thumb 

 and index finger are so furnished. Yet in the adults these 

 claws are generally wanting at least in the index finger. But 

 in the Ducks, for example, and in many birds of prey and in 

 the Water-hens, a claw will be found on both digits. In the 

 aberrant Opisthocomus these claws are large and functional 

 during the nestling period (p. 240), but disappear entirely during 

 adolescence. In the hind-limb of the embryo, similarly, there 

 will be found traces of five digits, while only four persist in the 



