sheep,
niata cattle, &c., are instances of the long-continued inheritance of rare
deviations of structure.

When we reflect that certain extraordinary peculiarities have {5} thus
appeared in a single individual out of many millions, all exposed in the
same country to the same general conditions of life, and, again, that the
same extraordinary peculiarity has sometimes appeared in individuals living
under widely different conditions of life, we are driven to conclude that
such peculiarities are not directly due to the action of the surrounding
conditions, but to unknown laws acting on the organisation or constitution
of the individual;--that their production stands in hardly closer relation
to the conditions than does life itself. If this be so, and the occurrence
of the same unusual character in the child and parent cannot be attributed
to both having been exposed to the same unusual conditions, then the
following problem is worth consideration, as showing that the result cannot
be due, as some authors have supposed, to mere coincidence, but must be
consequent on the members of the same family inheriting something in common
in their constitution. Let it be assumed that, in a large population, a
particular affection occurs on an average in one out of a million, so that
the _a priori_ chance that an individual taken at random will be so
affected is only one in a million. Let the population consist of sixty
millions, composed, we will assume, of ten million families, each
containing six members. On these data, Professor Stokes has calculated for
me that the odds will be no less than 8333 millions to 1 that in the ten
million families there will not be even a single family in which one parent
and two children will be affected by the peculiarity in question. But
numerous cases could be given, in which several children have been affected
by the same rare peculiarity with one of their parents; and in this case,
more especially if the grandchildren be included in the calculation, the
odds against mere coincidence become something prodigious, almost beyond
enumeration.

In some respects the evidence of inheritance is more striking when we
consider the reappearance of trifling peculiarities. Dr. Hodgkin formerly
told me of an English family in which, for many generations, some members
had a single lock differently coloured from the rest of the hair. I knew an
Irish gentleman, who, on the right side of his head, had a small white