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.—TExternal 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. 
Fia. 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: J, in blue light; IJ, in 
mixed white light; IJZ, 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 
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