hat complicated, and is shown
in the accompanying scheme. Essentially it lay in {66} following the
results flowing from crosses between blacks and whites. Experiments
recently made by Staples-Browne have shown that this case of reversion also
can be readily interpreted in Mendelian terms. In these experiments the
cross was made between black barbs and white fantails. The F_1 birds were
all black with some white splashes, evidently due to a separate factor
introduced by the fantail. On breeding these blacks together they gave an
F_2 generation, consisting of blacks (with or without white splashes),
blues (with or without white splashes), and whites in the ratio 9 : 3 : 4.
The factors concerned are colour (C), in the absence of {67} which a bird
is white, and a black modifier (B), in the absence of which a coloured bird
is blue. The original black barb contained both of these factors, being in
constitution CCBB. The fantail, however, contained neither, and was
constitutionally ccbb. The F_1 birds produced by crossing were in
constitution CcBb, and being heterozygous for two factors produced in equal
numbers the four sorts of gametes CB, Cb, cB, cb. The results of two such
series of gametes being brought together are shown in the usual way in Fig.
11. A blue is a bird containing the colour factor but lacking the black
modifier, _i.e._ of the constitution CCbb, or Ccbb, and such birds as the
figure shows appear in the F_2 generation on the average three times out of
sixteen. Reversion here comes about in F_2, when the redistribution of the
factors leads to the formation of zygotes containing one of the two factors
but not the other.

+-------+-------+-------+-------+
|CB#####|CB#####|CB#####|CB#####|
|CB#####|Cb#####|cB#####|cb#####|
|#|#|#|#|
|##BLACK|##BLACK|##BLACK|##BLACK|
+-------+-------+-------+-------+
|Cb#####|Cb.....|Cb#####|Cb.....|
|CB#####|Cb.....|cB#####|cb.....|
|#|.......|#|.......|
|##BLACK|...Blue|##BLACK|...Blue|
+-------+-------+-------+-------+
|cB#####|cB#####|cB |cB |
|CB#####|Cb#####|cB |cb |
|#|#| | |
|##BLACK|##BLACK| | |
+-------+-------+-------+-------+
|cb#####|cb.....|cb |cb |
|CB#####|Cb.....|cB |cb |
|#|.......| | |
|##BLACK|...Blue| | |
+-------+-------+-------+-------+

FIG. 11.

Diagram to illustr