ii] HEREDITY 9 



extent from very small to considerable range. They 

 therefore founded their theory on this type of 

 variability rather than on the occurrence of con- 

 siderable 'occasional variations' which are not 

 connected with the type by a series of intermediates. 

 It was not, however, until after the theory of 

 Natural Selection had obtained general recognition, 

 that any detailed study was undertaken of the actual 

 frequency and extent of variation, and its mode of 

 occurrence. 



The accurate investigation of variation has thus 

 been in progress only for some twenty or twenty-five 

 years, and according to the methods adopted students 

 have become divided into two somewhat distinct 

 schools. One of these has devoted itself rather to 

 the attempt to observe and classify the different 

 kinds of variation, and the other, generally called 

 the ' biometrician ' school, to measure its frequency 

 and range. It will be convenient to consider the 

 results obtained by the second method first. 



If a character is chosen which can be accurately 

 measured, such as human stature, and a sufficiently 

 large number of individuals are observed, it will 

 commonly be found that there is considerable range 

 of variation, and that every gradation in size occurs 

 between the smallest and largest. Such variation is 

 spoken of as ' continuous, 5 as opposed to ' discontinu- 

 ous' variation in which individuals of two kinds occur, 



