620 D. T. MACDOUGAL 
moist conditions would be a process resulting in enormous loss of 
species. Some spinose types would seem to offer the best morpho- 
logical features for such a change. 
Perhaps the most inportant of all of the altered conditions brought 
about by increasing moisture, however, would be the total trans- 
formation of the competitive struggle for existence. Animals would 
no longer play the predominating role as in arid areas. The num- 
ber of individuals representing the constituent species of a flora 
would be multiplied a hundred fold, perhaps a thousand fold, and 
once more the amount of food material offered to animals would 
decrease their total importance as a factor in selection, while the 
intensest crowding between roots and between shoots would once 
more be resumed and horizontal differentiation of associations such 
as that in forests would ensue. 
The element of a desert flora which would respond most readily 
to ameliorated aridity would, of course, be the hygrophytic annuals 
and perennials, which had survived the period of desiccation in 
their refuge of the rainy seasons, and in the moist areas along stream- 
ways and on elevated peaks. These would quickly occupy the greater 
part of the surfaces available for plants to the great intensification 
of the inter-vegetal struggle for existence. As these hygrophytes 
survived in the moist situations and the moist seasons of an arid 
period, so the surviving xerophytes in a moist period would find 
refuge in restricted habitats on talus slopes, rocks, and sand in which 
the soil-moisture relations would be best suited to their specialized 
structure and might display their seasonal activity during a period 
of the year in which the precipitation was least. 
Briefly restating the principal ideas touched upon, it may be said 
that Chamberlin’s prothesis of the planetesimal aggregation of the 
outer portions of the earth and the attendant conditions, together 
with current theories as to the catalytic nature of the essential activi- 
ties of protoplasm, makes possible rational speculations upon the 
origination of self-organizing matter. 
The passing of nitrates, phosphides, carbides, and sulphides into 
more stable combinations might readily result in the formation of 
thermo-catalysts, one type of which survived in the later forms of 
living matter. Similar combinations do not appear to be taking 
