The intervals between the horizontal lines in the diagram, may represent each a thousand or more generations.

After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a1 and m1.

These two varieties will generally still be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary; consequently they will likewise tend to vary, and commonly in nearly the same manner as did their parents.