118 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
formation, reduction divisions, and fertilization may ensue in tempera- 
tures 40° or 50° higher, a difference capable of being endured by the 
shoots of some plants now being tested, and which might well cause irre- 
versible developmental changes. Other factors of the environment may 
operate in a similar manner. 
It is therefore to be seen that all of the available evidence of a direct 
character tends to show that the alterations of shoots and vegetative 
members in general in response to the stimulation of external factors 
may or may not be of such nature as to result in increased fitness to the 
conditions concerned. Also, that such changes are not fully transmis- 
sible to succeeding generations and are in no sense irreversible. Adap- 
tation, therefore, furnishes but an insecure basis upon which to found 
a theory of the origin and development of any flora, inclusive of those 
characterizing arid regions. 
The influence of external conditions upon the germ plasm, however, 
has been seen to produce irreversible changes in a hereditary line by 
which new combinations of qualities and new characters were called 
out, which were fully transmissible. Furthermore, the newly produced 
forms perished in some localities endured by the parental type, but 
exceeded it in weathering the conditions of other localities. If to these 
facts it is added that one of such induced types which has been studied 
most thoroughly does not readily hybridize with the parent, even when 
growing with the branches interlocked, it is to be seen that the possi- 
bilities of origination and survival of new forms in any locality are very 
great. 
‘ Newly induced forms thrown into the complex conditions of an envi- 
ronment or locality do not necessarily survive because of qualities which 
fit them for competing with and crowding out other forms, but rather 
by reason of capacities suitable for the physical factors encountered. 
By reason of this they may, in reality, simply occupy a vacant niche 
in the system of living things present. The features of the competition 
ensuing when a new form is projected into the dense associations of 
mesophytic zones are lacking to some extent in deserts, since not so 
much crowding ensues as to the utilization of light and food-material. 
The chief struggle is not primarily among the various species of xeroph- 
ilous plants, but is waged between plants and animals over the water- 
supply. During the seasons of greater precipitation animals find food 
and water in plenty with but little disturbance to the plants which are 
essentially desert forms. With the coming of the arid seasons accom- 
panied by the death of the forms which furnish much of the food and 
the diminution of the water-supply, the animals are driven to make use 
of every possible source of water and food. One resource is to be found 
in the tender, turgid seedlings of the succulents which are eaten by the 
millions in their unprotected condition. If it were possible to exclude 
