wing
tail-feathers and hackles; and others in which there is no difference in
colour between the two sexes. In some cases the barred plumage, which in
gallinaceous birds is commonly the attribute of the hen, has been
transferred to the cock, as in the cuckoo sub-breeds. In other cases
masculine characters have been partly transferred to the female, as with
the splendid plumage of the golden-spangled Hamburgh hen, the enlarged comb
of the Spanish hen, the pugnacious disposition of the Game hen, and as in
the well-developed spurs which occasionally appear in the hens of various
breeds. In Polish fowls both sexes are ornamented with a topknot, that of
the male being formed of hackle-like feathers, and this is a new male
character in the genus Gallus. On the whole, as far as I can judge, new
characters are more apt {75} to appear in the males of our domesticated
animals than in the females, and afterwards to be either exclusively or
more strongly inherited by the males. Finally, in accordance with the
principle of inheritance as limited by sex, the appearance of secondary
sexual characters in natural species offers no especial difficulty, and
their subsequent increase and modification, if of any service to the
species, would follow through that form of selection which in my 'Origin of
Species' I have called sexual selection.

_Inheritance at corresponding periods of Life._

This is an important subject. Since the publication of my 'Origin of
Species,' I have seen no reason to doubt the truth of the explanation there
given of perhaps the most remarkable of all the facts in biology, namely,
the difference between the embryo and the adult animal. The explanation is,
that variations do not necessarily or generally occur at a very early
period of embryonic growth, and that such variations are inherited at a
corresponding age. As a consequence of this the embryo, even when the
parent-form undergoes a great amount of modification, is left only slightly
modified; and the embryos of widely-different animals which are descended
from a common progenitor remain in many important respects like each other
and their common progenitor. We can thus understand why embryology should
throw a flood of light on the natural system of classification, for this
ought to be as far as possible genealogical. When the embryo leads an
independent life, that is, becomes a larva, it has to be adapted to the
surrounding conditions in its structure and insti