Vegetation oe the Sotol Country. 
107 
Presumably the phenomena of adaptation are now in play as per- 
ceptibly as at any previous period, just as, in general, we assume 
that species are originating to-day as perceptibly as in previous 
ages under equally stimulating environments. Thus the peculiar 
fitness of cactus types, of Yucca, of Covilliaea, etc., as desert forms, 
and the cause of their being what they are structurally, night be 
discovered after measuring exactly the moisture conditions of soil 
and air confronting a given plant during the course of its devel- 
opment; also the temperatures and temperature variations in air 
and soil, the intensity and duration of illumination and variations 
in these; the chemical content of the soil and physical structure 
as affecting aeration, moisture, temperature, etc., and the response 
of the plant to these as expressed in its activity or passivity with 
respect to the absorption of water and mineral salts, and its dispo- 
sition of these elements; with respect to Carbon assimilation and 
subsequent metabolic processes; its behavior as to growth and re- 
production. All this means that valuable contributions to the life 
of desert plants can be made only after the application of as 
fine technique and as fine instruments and as much well directed 
study as is given in modern laboratories to other phases of plant 
life. 
Without attempting, therefore, to present exact measurements 
of the environmental factors in this connection, a general state- 
ment of them is here presented, applicable especially to the region 
lying between Devil’s Eiver and Sanderson. 
Rainfall. Within the belt just referred to, the average annual 
rainfall appears to be from 12 to 15 inches. The annual fall at 
Sanderson for 1901 was 7.30 inches and for 1902, 0.24 inches. It 
appears not to be exceptional for the entire year’s rainfall to fall 
below 6 inches. For the most part, the rain comes in sudden 
rather local storms so that the off-flow carries away the greater part 
of it. The seasonal distribution of rainfall is of special moment 
in this case. The usual season for the renewal of vegetation, — 
March and April — is apt to be quite rainless, the greater part of 
the season’s fall coming between June and October — especially in 
August and September, so that the coming of warm spring weather 
is not necessarily attended with luxuriant vegetative growth. It 
is said that more cattle die from weakness and starvation in March 
and April than in January and February because forage and water 
are scarce. 
Humidity. The normal annual relative humidity of .he atmos- 
phere decreases from 80.4 per cent at Galveston to about 55 per 
cent at Sanderson and 38.4 per cent at El Paso, at practically equal 
