134 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
changed but little since it became established in its present pes pre- 
sumably as late as the Pleistocene period. 
GENERAL MOVEMENTS. 
During the whole of this study, which has been given chiefly to plants 
of the Laboratory domain, the wider problems of distribution have not 
been lost sight of, but thus far the available data have not been suffi- 
cient to justify any final statement of general conclusions. Several con- 
tributions by specialists, however, have presented much that is of value 
as a solid foundation for future work, and that points toward conclu- 
sions which may be adopted provisionally as a basis for more extended 
investigation. 
The geological history, by Professor Tolman, has an important bearing 
on the subject of plant distribution in the region in which the Desert 
Laboratory is situated, and although the relations brought out are largely 
of a general character, they are not on this account the less significant. 
(1) The relatively recent origin of Tumamoc Hill, and the general con- 
tinuity of its history since the various igneous outbreaks that have been 
described, render it probable that, in many cases at least, the elements 
of its flora came to it in possession of essentially the same characters 
as they exhibit at the present day. That such is the case on certain 
parts of the hill is rendered evident by what is seen where topographic 
changes that have been described are now taking place, and the new 
areas are being occupied by representatives of species already estab- 
lished in their neighborhood. 
It is safe to assume that the continuity of geological history has been 
paralleled by a like continuity of plant life, and that precisely as the 
slow progression of geological events is best understood by exact study 
of what is now taking place, so the distribution and movements of the 
plants of the Laboratory domain, through the period of existence of 
Tumamoc Hill, are best studied by obtaining a clear conception of what 
they are at the present day. ‘This principle, once admitted, indicates a 
safe and fruitful method of procedure. 
(2) The leading topographical features, which are a direct outcome of 
the forces and events described, are conspicuously correlated with clearly 
recognizable associations of plants. ‘The rock surface of the hill, the 
slopes below, and the alluvial plain of the Santa Cruz are so many diverse 
habitats, each with its distinctive associations of plants. The several topo- 
graphic areas maintain their relative positions, but are continually under- 
going changes due to erosion and deposition, and these are succeeded by 
movements of the plant associations, so gradually that the relations of the 
associations to each other are hardly disturbed. But with the topograph- 
ical features as described, there necessarily exist differences of gradient 
and aspect, and these latter especially are accompanied by wide differences 
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