EXPERIMENTAL EVOLUTION I45 



(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 Allionia linearis (fig. 40) and Bahia dissecta 

 ^fi'- 33)- The diplophyll, which is characterized by a central band of 

 sponge tissue or storag-e cells, is found in Mcrtensia linearis (fig. 34). The 

 form of the spongophyll that is found in certain monocotyledons is shown 

 by Gyrosiachys 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 Qnercn-s (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 \he 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 io 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 



