78 HEREDITY AND VARIATION 



been taken advantage of in breeding sheep with long fleece, 

 Japanese cocks with tails ten feet long, " wonder horses " with 

 manes reaching the ground, and so on. 



But the offspring is sometimes so different from the parent that 

 we cannot describe its peculiarity as an incompleteness in the 

 expression of the normal inheritance, or as an exaggeration of 

 parental or ancestral traits. It is sometimes a new pattern, a 

 fresh departure, with what one might call organic originality. 

 It is more than a discontinuous variation. It seems to have 

 passed suddenly into a new position of organic equilibrium, 

 where it has not only individuality, but a distinctively novel 

 individuality. These distinctive novelties, which arise brusquely, 

 are often included in the conception of " mutations." 



§ 5. Fluctuating Variations 



When we examine a number of individuals of the same species 

 we usually find that they differ from one another in detail. 

 Some of the observed differences may be modificational or due 

 to differences of nurture, but it is often possible to abstract 

 these from differences due to hereditary nature. Thus, when 

 we collect a large number of specimens of the same age from the 

 same place at the same time, we often find that no two are 

 exactly alike. They have peculiarities of germinal origin — or, in 

 other words, they show fluctuating variations. The characteristic 

 feature of these fluctuations is that they are continuous, i.e. con- 

 nected by intergrades, and that they can be arranged in a gradual 

 series (a curve of frequency) on each side of a mode. 



To construct such a curve (let us say of variation in stature), 

 take a base line, and divide it into equal parts, each to represent 

 a unit of measurement, say an inch. From a middle division of 

 this base line erect an ordinate to represent by its length the 

 number of those individuals whose stature is found to be the 

 most frequent. On each side of this, from their appropriate 



