rely
white (with blue eyes) were, like her, invariably deaf; while those that
had the least speck of colour on their fur, as invariably possessed the
usual faculty of hearing."[824] The Rev. W. Darwin Fox informs me that he
has seen more than a dozen instances of this correlation in English,
Persian, and Danish cats; but he adds "that, if one eye, as I have several
times observed, be not blue, the cat hears. On the other hand, I have never
seen a white cat with eyes of the common colour that was deaf." In France
Dr. Sichel[825] has observed during twenty years similar facts; he adds the
remarkable case of the iris beginning, at the end of four months, to grow
dark-coloured, and then the cat first began to hear.

This case of correlation in cats has struck many persons as marvellous.
There is nothing unusual in the relation between blue eyes and white fur;
and we have already seen that the organs of sight and hearing are often
simultaneously affected. In the present instance the cause probably lies in
a slight arrest of development in the nervous system in connection with the
sense-organs. Kittens during the first nine days, whilst their eyes are
closed, appear to be completely deaf; I have made a great clanging noise
with a poker and shovel close to their heads, both when they were asleep
and awake, without producing any effect. The trial must not be made by
shouting close to their ears, for they are, even when asleep, extremely
sensitive to a breath of air. Now, as long as the eyes continue closed, the
iris is no doubt blue, for in all the kittens which I have seen this colour
remains for some time after the eyelids open. Hence, if we suppose the
development of the organs of sight and hearing to be arrested at the stage
of the closed eyelids, the eyes would {330} remain permanently blue and the
ears would be incapable of perceiving sound; and we should thus understand
this curious case. As, however, the colour of the fur is determined long
before birth, and as the blueness of the eyes and the whiteness of the fur
are obviously connected, we must believe that some primary cause acts at an
early period.

The instances of correlated variability hitherto given have been chiefly
drawn from the animal kingdom, and we will now turn to plants. Leaves,
sepals, petals, stamens, and pistils are all homologous. In double flowers
we see that the stamens and pistils vary in the same manner, and assume the
form and colour of the peta