er animals in the
number exceeding five; for no mammal, bird, existing reptile, or amphibian
(unless the tubercle on the hind feet of the toad and other tailless
Batrachians be viewed as a digit) has more than five; whilst fishes
sometimes have in their pectoral fins as many as twenty metacarpal and
phalangeal bones, which, together with the bony filaments, apparently
represent our digits with their nails. So, again, in certain extinct
reptiles, namely, the Ichthyopterygia, "the digits may be seven, eight, or
nine in number, a significant mark," says Professor Owen, "of piscine
affinity."[39]

We encounter much difficulty in attempting to reduce these various facts to
any rule or law. The inconstant number of the additional digits--their
irregular attachment to either the inner or outer margin of the hand--the
gradation which can be traced from a mere loose rudiment of a single digit
to a completely double hand--the occasional appearance of additional digits
in the salamander after a limb has been amputated--these various facts
appear to indicate mere fluctuating monstrosity; and this perhaps is all
that can be safely said. Nevertheless, as supernumerary digits in the
higher animals, from their power of regrowth and from the number thus
acquired exceeding five, partake of the nature of the digits in the lower
vertebrate animals;--as they occur by no means rarely, and are transmitted
with remarkable strength, though perhaps not more strongly than some other
anomalies;--and as with animals which have fewer than five digits, when an
additional one appears it is generally due to the development of a visible
rudiment;--we are led in all cases to suspect, that, although no actual
rudiment can be detected, yet that a latent tendency to the formation of an
additional digit exists in all mammals, including man. On this view, as we
shall more plainly see in the {17} next chapter when discussing latent
tendencies, we should have to look at the whole case as one of reversion to
an enormously remote, lowly-organised, and multidigitate progenitor.

* * * * *

I may here allude to a class of facts closely allied to, but somewhat
different from, ordinary cases of inheritance. Sir H. Holland[40] states
that brothers and sisters of the same family are frequently affected, often
at about the same age, by the same peculiar disease, not known to have
previously occurred in the family. He specifies the occurre