reeds. I can throw no light on these
differences in the power of hereditary transmission. Breeders believe,
and apparently with good cause, that a peculiarity generally becomes
more firmly implanted after having passed through several generations;
that is if one offspring out of twenty inherits a peculiarity from its
parents, then its descendants will tend to transmit this peculiarity to
a larger proportion than one in twenty; and so on in succeeding
generations. I have said nothing about mental peculiarities being
inheritable for I reserve this subject for a separate chapter.

{191} See _Origin_, Ed. i. p. 13.

{192} _Origin_, Ed. i. p. 86, vi. p. 105.

{193} It is interesting to find that though the author, like his
contemporaries, believed in the inheritance of acquired characters,
he excluded the case of mutilation.

_Causes of Variation._

Attention must here be drawn to an important distinction in the first
origin or appearance of varieties: when we see an animal highly kept
producing offspring with an hereditary tendency to early maturity and
fatness; when we see the wild-duck and Australian dog always becoming,
when bred for one or a few generations in confinement, mottled in their
colours; when we see people living in certain districts or circumstances
becoming subject to an hereditary taint to certain organic diseases, as
consumption or plica polonica,--we naturally attribute such changes to
the direct effect of known or unknown agencies acting for one or more
generations on the parents. It is probable that a multitude of
peculiarities may be thus directly caused by unknown external agencies.
But in breeds, characterized by an extra limb or claw, as in certain
fowls and dogs; by an extra joint in the vertebrae; by the loss of a
part, as the tail; by the substitution of a tuft of feathers for a comb
in certain poultry; and in a multitude of other cases, we can hardly
attribute these peculiarities directly to external influences, but
indirectly to the laws of embryonic growth and of reproduction. When we
see a multitude of varieties (as has often been the case, where a cross
has been carefully guarded against) produced from seeds matured in the
very same capsule{194}, with the male and female principle nourished
from the same roots and necessarily exposed to the same external
influences; we cannot believe that the endless slight differences
between seedling varieties thus produced