Species distribution can be understood through at least one factor.
 Note the following about the complex constant factors attached to some of the terms:The factor                                                         a              y              +              b              z                                      a              +              b                                            {\displaystyle {\frac {ay+bz}{a+b}}}   has the form of a weighted average of y and z
 When the outcome is produced by many small effects acting additively and independently, its distribution will be close to normal

