Palaeontology. 173 



earliest of all were almost certainly like Fig. 70 ; then they be- 

 came like Fig. 69 ; and, finally, only much later in geological 

 history (Jurassic or Cretaceous), they became like Fig. 68. This 

 order of change is still retained in the embryonic development 

 of the last introduced and most specialized order of existing 

 fishes. The family history is repeated in the individual history. 

 Similar changes have taken place in the form and structure of 

 birds' tails. The earliest bird known the Jurassic Archceo- 

 pteryx\i?i& a long reptilian tail of twenty-one joints, each joint 

 bearing a feather on each side, right and left (Fig. 71) : [see also 

 Fig- 73J- In the typical modern bird, on the contrary, the tail- 

 joints are diminished in number, shortened up, and enlarged, 

 and give out long feathers, fan-like, to form the so-ca'.led tail 

 (Fig. 72). The Archceoptcryx' tail is vertebrated^ the typical 

 bird's non-vertebr<i1ed. This shortening up of the tail did not 

 take place at once, but gradually. The Cretaceous birds, inter- 

 mediate in time, had tails intermediate in structure. The Hes- 

 perornis of Marsh had twelve joints. At first in Jurassic strata 

 the tail is fully a half of the whole vertebral column. It then grad- 

 ually shortens up until it becomes the aborted organ of typical 

 modern birds. Now, in embryonic development, the tail of the 

 modern typical bird passes through all these stages. At first the 

 tail is nearly one half the whole vertebral column ; then, as de- 

 velopment goes on, while the rest of the body grows, the growth 

 of the tail stops, and thus finally becomes the aborted organ we 

 now find. The ontogeny still passes through the stages of the 

 phylogeny. The same is true of all tailless animals. 



The extinct Archczopteryx above alluded to presents 

 throughout its whole organization a most interesting 

 assemblage of " generalized characters." For example, 

 its teeth, and its still unreduced digits of the wings 

 (which, like those of the feet, are covered with scales), 

 refer us, with almost as much force as does the verte- 

 brated tail, to the Sauropsidian type or the trunk 

 from which birds and reptiles have diverged. 



We will next consider the palaeontological evidence 



