312 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
two sets of determining causes acting simultaneously. First, there 
are the specific hereditary determiners or genetic factors, which react 
with the other elements of the protoplasm and, under favorable 
circumstances, condition normal development. Second, there are all 
the conditions external to the cell which stimulate or inhibit proto- 
plasmic activity. These “developmental stimuli” are chemical and 
physical changes wrought by energy from without the organism or 
caused by its own physiological activities. Chemical stimuli are 
exerted mainly through the medium of the circulating liquid which 
surrounds each living cell. Normally this fluid contains the elements 
essential for maintenance of life as well as various waste products. 
It may also bear toxic substances that suppress or inhibit the cell 
functions and in higher animals it contains the secretions of the duct- 
less, sexual and other glands that profoundly affect development. 
Physical stimuli are exerted chiefly from without and upon the organ- 
ism as a whole. They include changes in temperature, light and 
density of medium, the effects of electric and radiant energy, force of 
gravity, etc. Obviously, so many interrelated causes acting simulta- 
neously, 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 
characters 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 illustra- 
tions of these effects with reference to particular environmental 
factors. 
1. Environment modifies development of inherited characters.— 
(a) Light and Function. Klebs reports the result 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. 51. 
Although the visible differences between the three plants were very 
pronounced the experiment was carried much farther. During 1905-6 
observations were made on the numbers of stamens in the flowers 
of plants similarly propagated under white, red, and blue light and 
under variations, conditions of temperature, moisture, and food. 
About 20,000 flowers were examined and six distinct types were found, 
according to the variation in number of stamens. These had the 
