192 Causes and Course of Organic Evolution 



green leaves have arisen on each, it can readily be shoTMi that 

 these move their surfaces, or even in many flowering plants 

 their entire leaves, through pathways that are compounded 

 resultants due to optimum lumic stimulus (heliotropism), to 

 gravic action (diageotropism), to supra maximum light action 

 or intensity (paraheliotropism), even in some cases to mechanic 

 and chemic stimidi. Each leaf then, so long as it is in a grow- 

 ing state (and in many "sensitive" leaves long after growth 

 has ceased) is constantly proenvironing a course that later 

 is accurately taken. 



It is scarcely overstating the case to say that the greater 

 number of movements performed throughout the entire range 

 of the Caryota are not simple movements due to a single direct 

 environal stimulus, but are resultants of linked together re- 

 sponses, while as will be shown below they become, by pro- 

 gressive and easily traced stages, of great complexity in the 

 higher insects and mammals. The far-reaching value and 

 increasing importance of proenvironment then, as an evolu- 

 tionary factor, becomes more and more apparent as one rises 

 in the organic scale, while it receives its most remarkable illus- 

 trations in the responses of superman during the decades that 

 are now passing. Added illustrations w^ll be presented in the 

 succeeding chapter, and in other chapters of this work. But 

 the writer hopes to issue a separate volume in which graded 

 phases of this law will be treated of from all the leading groups 

 of the Caryota. 



Our scanty physiological knowledge of blue-green x\caryota 

 and to some extent even of the colorless ones, such as the bac- 

 teria, prevents us speaking in certain terms as to proenviron- 

 ment amongst these. The mode of growth and the lines of 

 growth response, the often irregular shapes assumed, and the 

 irregularly heaped up, coiled, or aggregated states of the simpler 

 Blue-green Algae from Chroococcus and Merismopedia tlirough 

 Nostoc and Wollea to Madifjocoleus and Fischerella or Sfig- 

 onema would suggest that they show single, sim])le, and slow 

 responses to separate types of stimuli. But, as we pass from 

 the last to Schizothrixy LyngbyUy and Oscillator la, the growth 



