carries its condemnation within itself; but, even supposing such a
definition were, in form, tenable, the physiologist who should attempt to
apply it in Nature would soon find himself involved in great, if not
inextricable, difficulties. As we have said, it is indubitable that
offspring _tend_ to resemble the parental organism, but it is equally
true that the similarity attained never amounts to identity either in form
or in structure. There is always a certain amount of deviation, not only
from the precise characters of a single parent, but when, as in most
animals and many plants, the sexes are lodged in distinct individuals, from
an exact mean between the two parents. And indeed, on general principles,
this slight deviation seems as intelligible as the general similarity, if
we reflect how complex the co-operating "bundles of forces" are, and how
improbable it is that, in any case, their true resultant shall coincide
with any mean between the more obvious characters of the two parents.
Whatever be its cause, however, the co-existence of this tendency to minor
variation with the tendency to general similarity, is of vast importance in
its bearing on the question of the origin of species.

As a general rule, the extent to which an offspring differs from its parent
is slight enough; but, occasionally, the amount of difference is much more
strongly marked, and then the divergent offspring receives the name of a
Variety. Multitudes, of what there is every reason to believe are such
varieties, are known, but the origin of very few has been accurately
recorded, and of these we will select two as more especially illustrative
of the main features of variation. The first of them is that of the "Ancon"
or "Otter" sheep, of which a careful account is given by Colonel David
Humphreys, F.R.S., in a letter to Sir Joseph Banks, published in the
"Philosophical Transactions" for 1813. It appears that one Seth Wright, the
proprietor of a farm on the banks of the Charles River, in Massachusetts,
possessed a flock of fifteen ewes and a ram of the ordinary kind. In the