(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and
self-fertilised plants of Ipomoea purpurea in the ten generations; the
mean height of the crossed plants being taken as 100. On the right hand,
the mean heights of the crossed and self-fertilised plants of all the
generations taken together are shown (as eleven pairs of unequal
vertical lines.))

The mean height of the self-fertilised plants in each of the ten
generations is also shown in the diagram 2/1, that of the intercrossed
plants being taken at 100, and on the right side we see the relative
heights of the seventy-three intercrossed plants, and of the
seventy-three self-fertilised plants. The difference in height between
the crossed and self-fertilised plants will perhaps be best appreciated
by an illustration: If all the men in a country were on an average 6
feet high, and there were some families which had been long and closely
interbred, these would be almost dwarfs, their average height during ten
generations being only 4 feet 8 1/4 inches.

It should be especially observed that the average difference between the
crossed and self-fertilised plants is not due to a few of the former
having grown to an extraordinary height, or to a few of the
self-fertilised being extremely short, but to all the crossed plants
having surpassed their self-fertilised opponents, with the few following
exceptions. The first occurred in the sixth generation, in which the
plant named "Hero" appeared; two in the eighth generation, but the
self-fertilised plants in this generation were in an anomalous
condition, as they grew at first at an unusual rate and conquered for a
time the opposed crossed plants; and two exceptions in the ninth
generation, though one of these plants only equalled its crossed