CHAPTER VI 



MUTATIONS 



SECTION I DISTINCTION BETWEEN MUTATION 

 AND ORDINARY VARIATION 



The deviations from type heretofore considered are those of 

 individuals rather than of groups. Whether quantitative, sub- 

 stantive, meristic, or functional, they represent the fluctuations 

 of individual members of a species or a variety about the nor- 

 mal type of the race, not necessarily exhibiting any tendency 

 to depart permanently from that type. 



The study of these deviations shows that, while no two 

 individuals are alike, yet the departures of certain individuals 

 in one direction are compensated by departures of other indi- 

 viduals in the opposite direction. In other words, the members 

 of a race cluster closely about what may be called a center of 

 fluctuation, which is, in most cases, comparatively stationary. 

 Because of this fact we may have a relatively fixed type, 

 indicating a practically stationary race, even in the midst of 

 considerable individual deviation. 1 



Mutations, on the other hand, mark sudden and distinct 

 departures from type. The pendulum swings, but does not re- 

 turn. A new center of fluctuation is established, from which 

 individuals deviate in all directions as before. It is not that 

 the old center is abandoned, for the mass of individuals still 

 cluster about it as before, but it is that a new center is estab- 

 lished, about which a new group clusters, and all close observers 

 recognize at once that a new type has been born into the world 



1 This fact is extremely confusing, especially to animal breeders. In the midst 

 of wide variations and with but few individuals living at any one time, the breeder 

 is often unable to tell whether his general average (or type) is improving, retro- 

 grading, or standing still. This matter will be alluded to again under " Type and 

 Variability." 



no 



