CHAPTER VIII 



INTERNAL CAUSES OF VARIATION 



While the causes of variation are both internal and external 

 to the organism, the facts of the last chapter must satisfy 

 the student of breeding problems that many of the processes 

 attendant upon growth and reproduction are fruitful sources of 

 variability. It is the purpose of the present chapter to discuss 

 these internal influences somewhat at length. They are of two 

 kinds, (i) those affecting the individual only, and (2) those 

 affecting the race as a whole. It is expedient to distinguish 

 between these two classes, and the chapter will be divided into 

 two parts, corresponding to these distinctions, as follows: (i) 

 internal influences affecting primarily the individual ; (2) internal 

 influences affecting the race as a whole. 



I INTERNAL INFLUENCES AFFECTING PRIMARILY 

 THE INDIVIDUAL 



SECTION I CELL DIVISION 



Growth is the result of cell division. Manifestly, therefore, 

 all differences in size or in pattern are intimately dependent upon 

 the extent and regularity of this process. 



Morphological variation due to cell division. Whatever influ- 

 ences underlie the phenomena of mitosis, all questions of form 

 or size are absolutely dependent upon the extent to which cell 

 division and its attendant growth proceed. The individual cells 

 in giants are not larger than those of normal specimens, but they 

 are more numerous ; and in dwarfs they are not smaller, but 

 fewer in number. 1 What energies decide how far cell division 

 shall proceed and when it shall stop in the case of each separate 



i Wilson, The Cell, pp. 388-389. 

 '55 



