VARIATION 



21 



interrelated causes acting simultaneously, each being independently 

 capable of inducing a change in the end product, may cause an infinite 

 number of differences in substance and in degree of development. 



Variation and Environment. External stimuli affect the develop- 

 ment of characters in three ways: (1) they modify the development of 

 inherited characters; (2) they actually condition the production of charac- 

 ters whose hereditary determiners are present in the germ-plasm; (3) 

 they may cause germinal variations which result in the appearance of 

 new heritable characters. The following are illustrations of these effects 

 with reference to particular environmental factors. 



FIG. 4. Sedum spectabile. The three shoots (taken from a single plant) were planted in 

 small pots on March 12, 1904, and placed in different greenhouses: /, in blue light; //, in 

 mixed white light; ///, in red light. Photographed on Sept. 30, 1914. (After Klebs.) 



1. Environment Modifies Development of Inherited Characters. 



(a) Light and Function, Klebs reports the results of growing the Showy 

 Sedum (Sedum spectabile) in white, red and blue light. The diverse 

 effects of the three kinds of light are clearly shown in Fig. 4. Although 

 the visible differences between the three plants were very pronounced 

 the experiment was carried much further. During 1905-06 observations 

 were made on the numbers of stamens in the flowers of plants similarly 

 propagated under white, red and blue light and under various conditions 

 of temperature, moisture, and food. About 20,000 flowers were examined 



