612 D. T. MACDOUGAL 
y 
greatly lessened and consequently the amount of humus would 
decrease; wind erosion would remove much of this from its place of 
formation and by this means alone the distribution of many species 
would be totally altered. The soil moisture would ultimately be 
so depleted that the surface layers would show as great a proportion 
as the underlying layers, carrying an excess during seasons of precipi- 
tation, a fact that would have the profoundest influence upon the 
native vegetation, determining not only the habit of the root-systems, 
the form of the shoot, but also becoming a factor in distribution, 
and giving a new form to the competitive struggle among the organ- 
isms in a locality. The change in precipitation would result in 
the formation of long outwash, detrital slopes, or bajadas, giving 
new habitat conditions, and a further differentiation would consist 
in the surface deposition of soil salts, giving alkaline and saline 
areas upon which halophytic, or saline plants flourish. The lessened 
relative humidity would result in modification of foliar surfaces, make 
necessary for survival special structures in seeds and spores, and 
would be followed by a more intense insolation by reason of the non- 
absorption of some portions of the spectrum, and lastly the course 
of the temperature of the soil would change with the depletion of 
the humus and the altered water relations. 
If desiccation ensued as a result of simple horizontal reduction 
of the precipitation, in a region with an unbroken surface lying at 
nearly the same level, the effect would be sweeping, monotonous, 
and with an almost total absence of selective effect that would mean 
extermination, or change in a flora en bloc. ‘The majority of inter- 
pretations of the paleontological record assume such results. It is 
to be seen, however, that desiccation in a region with diversified 
topography and great differences in level would result in great differ- 
entiation, and if to this reduction is added the restriction of the rain- 
fall to one or two brief seasons or to limited periods a maximum of 
effect may be expected. 
The development of desert conditions in the manner described 
over a region of any extent would entail the least disturbance on 
mountain summits, where, by reason of the lowered temperature and 
the facilities for condensation, the evaporating power of the air 
would remain lowest. The original, or pre-desert forms would be able 
