young
families there are always a few pied ones." These changes of plumage,
which appear and are inherited at various corresponding periods of life
in the pigeon, canary-bird, and rook, are remarkable, because the
parent-species undergo no such change.

Inherited diseases afford evidence in some respects of less value than
the foregoing cases, because diseases are not necessarily connected
with any change in structure; but in other respects of more value,
because the periods have been more carefully observed. Certain diseases
are communicated to the child apparently by a process like inoculation,
and the child is from the first affected; such cases may be here passed
over. Large classes of diseases usually appear at certain ages, such as
St. Vitus's dance in youth, consumption in early mid-life, gout later,
and apoplexy still later; and these are naturally inherited at the same
period. But even in diseases of this class, instances have been
recorded, as with St. Vitus's {78} dance, showing that an unusually
early or late tendency to the disease is inheritable.[169] In most
cases the appearance of any inherited disease is largely determined by
certain critical periods in each person's life, as well as by
unfavourable conditions. There are many other diseases, which are not
attached to any particular period, but which certainly tend to appear
in the child at about the same age at which the parent was first
attacked. An array of high authorities, ancient and modern, could be
given in support of this proposition. The illustrious Hunter believed
in it; and Piorry[170] cautions the physician to look closely to the
child at the period when any grave inheritable disease attacked the
parent. Dr. Prosper Lucas,[171] after collecting facts from every
source, asserts that affections of all kinds, though not related to any
particular period of life, tend to reappear in the offspring at
whatever period of life they first appeared in the progenitor.

As the subject is important, it may be well to give a few instances,
simply as illustrations, not as proof; for proof, recourse must be had
to the authorities above quoted. Some of the following cases have been
selected for the sake of showing that, when a slight departure from the
rule occurs, the child is affected somewhat earlier in life than the
p