ORIGIN OF DESERT FLORAS. 115 
semination must have occurred from surrounding regions. Neither the 
survivors nor the invaders, however, would include the peculiarly xero- 
phytic forms by which any desert is characterized. 
The problems involved in the study of the origination of these forms 
comprise all of the main questions as to the methods, procedure, and 
actuating causes in evolutionary movements, and are by no means to be 
solved within the brief limits of this paper. The known effects of some 
of the factors concerned and the major limitations of the whole subject 
may be profitably outlined, however. 
An adjustment of perspective may well be made before going farther 
into the qualities of desert vegetation. Most of the contact of the human 
race with plants, leading into a consideration of their intimate and gen- 
eral nature, has taken place in the climates of moist temperate and 
tropical zones, where the features encountered are unconsciously assumed 
to be normal, typical and average, while those presented by other regions, 
including alpine zones and desert areas, are regarded as adaptational, 
highly specialized and abnormal. ‘The coating of stems and leaves with 
wax, hairs or spines, is no more highly adaptational, however, than the 
formation of bark on trees, or leaf excision in deciduous plants, and 
scores of other features offered by species of the moist temperate regions, 
and in most respects they do not depart more widely from primitive 
morphological types. If the structures offered by plants of a moist 
forest of the Mississippi Valley had been first dealt with by the trained 
scientists of a desert-dwelling people, much would have been made of 
their non-typical and adaptive character. The fact is not to be lost 
sight of, however, that the general progress of conditions has been toward 
desiccation and not increased precipitation, and that in a general way 
the xerophyte is one of the recent developments of the vegetable kingdom. 
The relation existing between the structure, general form, and habits 
of an organism and the environmental conditions encountered by it are 
extremely difficult to decipher. In no branch of biological science are 
plausible explanations so easily framed, and in no interpretations of 
natural phenomena is it so easy to go astray. It seems to be tacitly 
assumed by the majority of writers that when a plant is taken into a 
new environment it undergoes changes which fit it better for that envi- 
ronment. This does not always follow. Thus, if a mesophyte is com- 
pelled to develop under arid conditions, the shoot formed will differ more 
or less markedly from that which might have been formed under the 
average conditions to which the species was accustomed. ‘This departure 
is in no sense an adaptation, but is the direct result of the balance 
established by the absorbing organs, the conducting stems, and the 
transpiratory surfaces with regard to the absorption, formation, trans- 
portation and transpiration of water and other food-material. Restricted 
surfaces of the shoots might be an advantage and offer increased suit- 
