Wright's ram or the hexadactyle members of Gratio Kelleia.

Varieties then arise we know not why; and it is more than probable that the
majority of varieties have arisen in this "spontaneous" manner, though we
are, of course, far from denying that they may be traced, in some cases, to
distinct external influences; which are assuredly competent to alter the
character of the tegumentary covering, to change colour, to increase or
diminish the size of muscles, to modify constitution, and, among plants, to
give rise to the metamorphosis of stamens into petals, and so forth. But
however they may have arisen, what especially interests us at present is,
to remark that, once in existence, many varieties obey the fundamental law
of reproduction that like tends to produce like; and their offspring
exemplify it by tending to exhibit the same deviation from the parental
stock as themselves. Indeed, there seems to be, in many instances, a
prepotent influence about a newly-arisen variety which gives it what one
may call an unfair advantage over the normal descendants from the same
stock. This is strikingly exemplified by the case of Gratio Kelleia, who
married a woman with the ordinary pentadactyle extremities, and had by her
four children, Salvator, George, Andre, and Marie. Of these children
Salvator, the eldest boy, had six fingers and six toes, like his father;
the second and third, also boys, had five fingers and five toes, like their
mother, though the hands and feet of George were slightly deformed. The
last, a girl, had five fingers and five toes, but the thumbs were slightly
deformed. The variety thus reproduced itself purely in the eldest, while
the normal type reproduced itself purely in the third, and almost purely in
the second and last: so that it would seem, at first, as if the normal type
were more powerful than the variety. But all these children grew up and
intermarried with normal wives and husband, and then, note what took place:
Salvator had four children, three of whom exhibited the hexadactyle members
of their grandfather and father, while the youngest had the pentadactyle