CHAPTER III 



THE CAUSES OF VARIATION 



In the last chapter the distinction has been 

 explained between continuous and discontinuous 

 variation ; some confusion has however arisen with 

 regard to the terms used in describing these conditions. 

 Continuous variation about a mean (or more accur- 

 ately modal) condition is sometimes spoken of as 

 ' fluctuation,' but as will be seen below this kind of 

 variability probably includes two very distinct groups 

 of facts. It may include inherent variability arising 

 in the germ-cells, or it may include differences in the 

 adult condition having their origin in different effects 

 of environment during growth. Some writers have 

 used the word ' fluctuation ' for this latter condition 

 only. 



Discontinuous variation is sometimes called 

 ' mutation,' a word which also has been used in 

 several senses. It may mean the appearance of a 

 form varying discontinuously from the type, or it 

 may be applied to the discontinuous character itself. 



