PEDIGREES AND FAMILY TIES. 211 



the embryo. Whale-bone whales have no teeth, 

 except in the embryo where small ones are re- 

 cognizable, yet small teeth appear in the embryo. 



The size of these vestigial teeth bears some 

 proportion to the time which has elapsed since 

 they were useful. They grow less and less as 

 the time since they were useful grows greater 

 and greater, till they finally cease to record 

 themselves any longer. It is as if these struc- 

 tures were originally endowed, so to speak, with 

 a certain amount of material set aside for their 

 support. So long as they were used this capital 

 or endowment gained interest, as soon as they 

 ceased to be used the interest stopped and the 

 capital slowly vanished. 



In the bird's wing we have a good illustration 

 of this. The rails are birds of feeble flight and 

 small wings, which in some have become so small 

 as to be rendered useless; in the apteryx they- 

 are so tiny as to require a careful search to find 

 them ; in the old fossil Hesperornis only the upper 

 arm-bone was left, and in the moas even this dis- 

 appeared. The first in time to become flightless 

 were the moas, the last some species of rails. In 

 the flightless species the wing is developed in 

 every young one that is born ; but in every 

 young one it is probably a shade smaller than 

 in its parents, and so it will go on dwindling in 

 each generation, at last appearing only in the 

 embryo, and finally disappearing even in these. 



The claw on the third finger of Archceq)terijx 

 was very probably only used by the nestling, 

 and perhaps also by the adult during the moulting 

 period. Soon the adult learnt to moult his quills 



