170 



HEREDITY AND EVOLUTION IN PLANTS 



they quickly perish unless they find at once suitable 

 conditions for germination and development. Thus they 

 could not spread rapidly over large areas. In the light 

 of these facts the only logical inference is that their 

 wide and discontinuous distribution must have required 

 a vast period of time. The tulip tree, represented now 

 by only one genus (Liriodendron) and one or possibly 



FIG. 78. Map showing the known geographical distribution of the 

 bald cypress (Taxodium) in the Tertiary and Pleistocene. Tertiary dis- 

 tribution, shaded; Pleistocene occurrences north of its present limits, in 

 dots; present distribution, black. (From Shimer, after E. W. Berry.) 



two species, and with discontinuous distribution (Eastern 

 North America and China), represents an old type now, 

 perhaps, on the way to extinction. A similar statement 

 may be made for Sassafras, for the bald cypress (Taxodium, 

 Fig. 78), and numerous other groups. 



In general it may be said that groups considered relatively 

 more primitive or ancient on morphological or paleonto- 



