276 



THE EVOLUTION THEORY 



of flowers in phanerogams. Tliese showed that tlie common balsam 

 {Iinjjat'iens noli riie tangere) produces its familiar open flowers in 

 a strong light, hut in weak light only bears small, closed, so-called 

 ' cleistogamous ' flowers. But it would be utterly erroneous to suppose 

 that the strong or weak light is the real cause, the causa onaterialis, 

 of these two forms of flowers; the degree of illumination is merely the 

 stimulus which provokes one or other of the primary constituents 

 to development, both kinds being present in the constitution of the 

 plant. As has long been known, the balsam normally possesses two 

 kinds of flowers, and the slumbering primary constituents of these 

 are so arranged that tlie open flowers develop where there is a prospect 



Fig. 129. ^1, an aberration of Arctla cajcu produced by low tcmi)erature. 

 B, the most divergent member of its i^rogeny. After E. Fischer. 



of insect visits and cross-fertilization, that is, in sunny weather or 

 in a strong light, while closed and inconspicuous flowers adapted for 

 self-fertilization develop in weak light, that is, in shady places and 

 in concealed parts of the plant, where insect visits are not to be 

 expected. 



Among plants we And thousands of instances of such reactions 

 of the organism to external stimulus —reactions which are not of 

 a primary nature, that is, are not the inevitable consequences of the 

 plant's constitution, but which depend upon adaptations of the special 

 constitution of a species or group of species to the specific conditions 

 of its life. To this category belong all the phenomena of heliotropism, 

 geotropism, and chemotropism, which have been discovered by the 



