Appendix B 375 



or in structure, between them and their nearest relatives first found in 

 the lower Miocene age, some twenty-three millions of years later. 



The Zeuglodonts, now totally extinct as far as we know, are definitely 

 more primitive than any existing known whales but, in some respects, 

 link all of them to their common terrestrial ancestors. Yet the Zeuglo- 

 donts are still whales and their history does not extend backwards beyond 

 the middle Eocene, some fifty million years ago. 



The accompanying chart displays the time sequence proportionately, 

 but the subdivisions of the major ages — of the Phocene, Miocene, Oligo- 

 cene, and Eocene — are not proportionate but simply diagrammatic. They 

 represent difi"ering periods of time in almost every case, but the exact 

 number of years by which they differ has not yet been adequately agreed 

 upon in all cases, and this precludes any definite pictorial breakdown and 

 renders any attempt at generalization unwarranted. Further, the arrival 

 upon the geologic scene or the disappearance therefrom of any form does 

 not necessarily occur at exactly the beginning or end of any one sub- 

 division of the periods as herein shown. Animals do not just spring spon- 

 taneously into existence, so each group must have had ancestors. Sim- 

 ilarly, they did not all just disappear overnight and we may assume that 

 they hngered on somewhere, though no longer leaving skeletons en- 

 tombed in the rocks, or doing so in strata that we have not yet investi- 

 gated. Substantially, this chart represents our present knowledge of the 

 past history of the whales, but it may well be greatly altered at any time 

 by new discoveries. 



