EXTERNAL INFLUENCES AS CAUSES OF VARIATION 239 



would be likely to develop decided geotropism ; or, conversely, 

 we may say that organisms with marked geotropism would be 

 likely to become fixed. In either event less geotropism would be 

 expected among animals, and either assumption would square 

 with the facts. Many lower organisms truly animal are distinctly 

 geotropic, however, and most animals show a decided preference 

 as to position with reference to gravity. Both with land animals, 

 high or low, and with fish as well, the ventral side is carried 

 downward, and the anterior portion in general upward. 



Effect of geotropism upon protoplasm. The protoplasm of cells, 

 plant or animal, is not homogeneous. The nucleus is heavier 

 than the cytoplasm, and together with chlorophyll granules and 

 starch grains tends to settle to the lower side of the cell, giving 

 it a kind of polarity due to gravity. " In many ova the yolk 

 sinks to the lower pole and the cytoplasm floats on top, in 

 whatever position the egg may be held," a fact which 

 " undoubtedly has an important effect upon development." 1 



General effect of gravity upon development. There is no room 

 for doubt as to the profound effect of gravity upon development. 

 However, this influence of gravity has been continual and con- 

 stant on all existing species for untold generations, and it may 

 be looked upon as having already exerted the maximum of its 

 influence upon all forms of life. 



The effect of gravity upon development has, therefore, long 

 ago reached the position of a constant force to be reckoned with, 

 and is now to be regarded as a fixed factor in development rather 

 than as a present cause of individual deviation, to be studied 

 more for the sake of learning the degree of dependence of liv- 

 ing matter upon outside forces rather than as a direct means of 

 further change. 



SECTION VI EFFECT OF LIGHT UPON LIVING MATTER 



In all chlorophyllaceous plants the amount of carbon fixed, 

 and therefore the total of growth in the sense of increase in dry 

 matter, is in almost direct proportion to the expanse of leaf sur- 

 face and the amount of light that falls upon it. 



1 C. B. Davenport, Experimental Morphology, Part I, p. 114. 



