1648 THE STORY OF THE UNIVERSE 



and iron stages of culture. Evidence camps, habi- 

 tations, tombs, refuse-heaps, surface accumulation, 

 caves, alluvia, peat bogs, submarine forests, raised 

 beaches. 



VI. Historic in which the events are recorded in 

 history. Evidence documents, refuse-heaps, caves, 

 tombs. 



The orders, families, genera, and species in the 

 above summary, when traced forward in time, fall 

 into the shape of a genealogical tree, with its trunk 

 hidden in the secondary period, and its branchlets 

 (the living species) passing upward from the Plio- 

 cene, a tree of life with living Mammalia for its 

 fruits and foliage. Were the extinct species taken 

 into account, it would be seen that they fill up the 

 intervals separating one living form from another, 

 and that they too grow more and more like the living 

 forms as they approach nearer to the present day. 

 It must be remembered that in the above definitions 

 the fossil marsupials are purposely ignored, because 

 they began their specialization in the secondary pe- 

 riod, and had arrived in the Eocene at the stage which 

 is marked by the presence of living genus the 

 opossum (Didelphys). 



It will be seen that our inquiry into the antiquity 

 of man is limited to the last four of the divisions. 

 The most specialized of all animals can not be looked 

 for until the higher Mammalia by which he is sur- 

 rounded were alive. We can not imagine him in the 

 Eocene age, at a time when animal life was not suffi- 

 ciently differentiated to present us with any living 

 genera of placental mammals. Nor is there any 



