And here the importance of the principle of benefit derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as would be thought worthy of record in a systematic work.
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.