618 D. T. MACDOUGAL 
make possible their maintenance as independent inhabitants of the 
dry land. 
It is true, of course, that desert conditions are not favorable for 
fossilization, yet many opportunities for such action undoubtedly 
occur in the carrying and burying action of the torrential floods of 
desert streamways, while wind-blown deposits might preserve the 
more indurated forms. Many of these and the skeletons of the 
Cactaceae would seem well adapted for preservation in this manner, 
although no remains have yet been uncovered. The view that such 
forms are of recent origin, within the present period of advancing 
desiccation, would predicate a very great phylogenetic activity unpre- 
cedented perhaps, but by no means impossible. 
The actual relationship between plants and their environment 
is by no means a settled question and since this and related problems 
are to be discussed in detail at the Darwin memorial session of this 
meeting, this subject will not be considered here farther than to say 
that it is unsafe to asssume that any organism has undergone adap- 
tation and fitting- specialization in direct somatogenic response to 
any set of environic factors, and that admissible evidence on such 
matters is extremely difficult to obtain." 
The operations of factors lessening the supply of water to any 
region would of course result in greater aridity in some places than 
in others and the movements of xerophytic forms established in 
these to other contiguous areas dried out later would be a matter 
in which the direction of the winds, streamways, movements of 
animals, and position of mountain barriers would play a determin- 
ing part. . 
The recession of large expanses of water included in a desiccating 
region, such as has occurred in the great basins in Utah and Nevada, 
and in the bolsons to the southward and eastward in New Mexico, 
Chihuahua, and Arizona would present special conditions. The rate 
at which the waters of such inland seas might recede, however, would 
be such that the advance of vegetation to cover the immersed areas 
would be quite as rapid as that necessary to follow a receding ice- 
sheet or a change of climate due to any cause. Thus our observa- 
tions on the Salton Lake show that beaches a mile in width are bared 
« Fifty Years of Darwinism. New York: H. Holt & Co., 1909. 
