EXPERIMENTAL EVOLUTION 145 
(fig. 32). Plants with isophotophylls are found chiefly in xerophytic places, 
though erect leaves of this type occur in most sunny habitats. The 
staurophyll, in which the protection is due to the extreme development of 
palisade tissue, is illustrated by Alliomta linearis (fig. 40) and Bahia dissecta 
(fig. 33). The diplophyll, which is characterized by a central band of 
sponge tissue or storage cells, is found in Mertensia linearis (fig. 34). The 
form of the spongophyll that is found in certain monocotyledons is shown 
by Gyrostachys stricta (fig. 45). The spongophyll (fig. 38:3, 39:2) is 
frequent among plants of deep shade, but as the leaf sections of Allionia 
(figs. 38, 40) and Quercus (fig. 42) show, the diphotophyll.is the rule in 
shade ecads. 
EXPERIMENTAL EvoLuTIon 
183. Scope. The primary task of experimental evolution is the de- 
tailed study, under measured conditions, of the origin of new forms in 
nature. As a department of botanical research that is as yet unformed, it 
has little concern with the host of hypotheses and theories which rest 
merely upon general observation and conjecture. A few of these constitute 
_ good working hypotheses or serve to indicate possible points of attack, but 
the vast majority are worthless impedimenta which should be thrown away 
at the start. It is the general practice to speak of evolution as founded upon 
a solid basis of incontestible facts, but a cursory examination of the evidence 
shows that it is drawn, almost without exception, from observaton alone, 
and has in consequence suffered severely from interpretation. With the 
exception of DeVries’s work on mutation, sustained and accurate investiga- 
tion of the evolution of plants ‘has been lacking. As a result, botanical 
research has been built high upon an insecure foundation, nearly every stone 
of which must be carefully tested before it can be left permanently in place. 
In a field so vast and important as evolution, experiment should far outrun 
induction, and deduction should enter only when it can show the way to a 
working hypothesis of real merit. The great value of DeVries’s study of 
mutation as an example of the proper experimental study of evolution has 
been seriously reduced by the fact that the “mutation theory” has carried 
induction far beyond the warrant afforded by experiment. The investigator 
who plans to make a serious study by experiment of the origin of new plant 
forms should rest secure in the conviction that the most rapid and certain 
progress can be made only by the accumulation of a large number of un- 
impeachable facts, obtained by the most exact methods of experimental 
study. 
The general application of field experiment to evolution will render the 
current methods of recognizing species quite useless. It will become im- 
perative to establish an experimental test for forms and species, and to 
