fine our
attention to the coloured types and to the Himalayans. The proportion in
which the four coloured types appeared in F_2 was very nearly 9 agoutis, 3
blacks, 3 yellows, and 1 tortoiseshell. Evidently we are here dealing with
two factors: (1) the grey factor (G), which modifies black into agouti, or
tortoiseshell into yellow; and (2) an intensifying factor (I), which
intensifies yellow into agouti and tortoiseshell into black. It may be
mentioned here that other experiments confirmed the view that the yellow
rabbit is a dilute agouti, and the tortoiseshell a dilute black. The
Himalayan pattern behaves as a recessive to self-colour. It is a
self-coloured black rabbit lacking a factor that allows the colour to
develop except in the points. That factor we may denote {62} by X, and as
far as it is concerned the Himalayan is constitutionally xx. The Himalayan
contains the intensifying factor, for such pigment as it possesses in the
points is full coloured. At the same time it is black, _i.e._ lacking in
the factor G. With regard to these three factors, therefore, the
constitution of the Himalayan is ggIIxx. The last character which we have
to consider in this cross is the Dutch character. This was found by Hurst
to behave as a recessive to self-colour (S), and for our present purpose we
will regard it as differing from a self-coloured rabbit in the lack of this
factor.[3] The Himalayan is really a self-coloured animal, which, however,
is unable to show itself as a full black owing to its not possessing the
factor X. The results of breeding experiments then suggest that we may
denote the Himalayan by the formula ggIIxxSS and the yellow Dutch by
GGiiXXss. Each lacks two of the factors upon the full complement of which
the agouti colour depends. By crossing them the complete series GIXS is
brought into the same zygote, and the result is a reversion to the colour
of the wild rabbit.

Bush x Cupid
|
Tall -------------------------- F_1
|
+----------+---+------+----------+
Tall Bush Cupid Cupid -------- F_2
(procumbent) (erect)

Most of the instances of reversion yet worked out are those in which colour
characters are concerned. The sweet pea, however, supplies us with a good
example of reversion in structural characters. A dwarf variety known as the
"Cupid" has been extensively grown for {63} some years. In these li