(eistocene: development of xeric environments PLIOCENE Y.N. Mex. plateau 1ove-estab'n. isthmiailink. 2uplift of upland areas (Mexican Plateau to Cuygwi) middle Amer. + old North types S.A. constriction of tropics by Mex. plateau to north) PLEISTOCENE A. lowland dispersal a) Atlantic lowland route b) Pacific dry-glass rows 1. northern 2. subhumid corridor 3. southern 1. Xeric forms (eg. Cuyricobolous cubens found S-10% of N.A.favw) Culto Dulce found dispersal to sargasso (found in sargasso islands) w/ very few xeric form even to Guayacaste Culto Dulce moist barrier upper lower bank P. coast outflow. 2. ( more C. Amer. flirty meeting south at S.A. elements ( north) Where c./ w/ more endemic form rather than mixin of N.L.S. elements