<h3>LAWS OF INHERITANCE.</h3>
<p>In order to understand how sexual selection has acted on many animals of many
classes, and in the course of ages has produced a conspicuous result, it is
necessary to bear in mind the laws of inheritance, as far as they are known.
Two distinct elements are included under the term
“inheritance”—the transmission, and the development of
characters; but as these generally go together, the distinction is often
overlooked. We see this distinction in those characters which are transmitted
through the early years of life, but are developed only at maturity or during
old age. We see the same distinction more clearly with secondary sexual
characters, for these are transmitted through both sexes, though developed in
one alone. That they are present in both sexes, is manifest when two species,
having strongly-marked sexual characters, are crossed, for each transmits the
characters proper to its own male and female sex to the hybrid offspring of
either sex. The same fact is likewise manifest, when characters proper to the
male are occasionally developed in the female when she grows old or becomes
diseased, as, for instance, when the common hen assumes the flowing
tail-feathers, hackles, comb, spurs, voice, and even pugnacity of the cock.
Conversely, the same thing is evident, more or less plainly, with castrated
males. Again, independently of old age or disease, characters are occasionally
transferred from the male to the female, as when, in certain breeds of the
fowl, spurs regularly appear in the young and healthy females. But in truth
they are simply developed in the female; for in every breed each detail in the
structure of the spur is transmitted through the female to her male offspring.
Many cases will hereafter be given, where the female exhibits, more or less
perfectly, characters proper to the male, in whom they must have been first
developed, and then transferred to the female. The converse case of the first
development of characters in the female and of transference to the male, is
less frequent; it will therefore be well to give one striking instance. With
bees the pollen-collecting apparatus is used by the female alone for gathering
pollen for the larvae, yet in most of the species it is partially developed in
the males to whom it is quite useless, and it is perfectly developed in the
males of Bombus or the humble-bee. (32. H. Muller, ‘Anwendung der
Darwin’schen Lehre,’ etc., Verh. d. n. V. Jahrg., xxix. p. 42.) As
not a single other Hymenopterous insect, not even the wasp, which is closely
allied to the bee, is provided with a pollen-collecting apparatus, we have no
grounds for supposing that male bees primordially collected pollen as well as
the females; although we have some reason to suspect that male mammals
primordially suckled their young as well as the females. Lastly, in all cases
of reversion, characters are transmitted through two, three, or many more
generations, and are then developed under certain unknown favourable
conditions. This important distinction between transmission and development
will be best kept in mind by the aid of the hypothesis of pangenesis. According
to this hypothesis, every unit or cell of the body throws off gemmules or
undeveloped atoms, which are transmitted to the offspring of both sexes, and
are multiplied by self-division. They may remain undeveloped during the early
years of life or during successive generations; and their development into
units or cells, like those from which they were derived, depends on their
affinity for, and union with other units or cells previously developed in the
due order of growth.</p>
<h3>INHERITANCE AT CORRESPONDING PERIODS OF LIFE.</h3>
<p>This tendency is well established. A new character, appearing in a young
animal, whether it lasts throughout life or is only transient, will, in
general, reappear in the offspring at the same age and last for the same time.
If, on the other hand, a new character appears at maturity, or even during old
age, it tends to reappear in the offspring at the same advanced age. When
deviations from this rule occur, the transmitted characters much oftener appear
before, than after the corresponding age. As I have dwelt on this subject
sufficiently in another work (33. The ‘Variation of Animals and Plants
under Domestication,’ vol. ii. 1868, p. 75. In the last chapter but one,
the provisional hypothesis of pangenesis, above alluded to, is fully
explained.), I will here merely give two or three instances, for the sake of
recalling the subject to the reader’s mind. In several breeds of the
Fowl, the down-covered chickens, the young birds in their first true plumage,
and the adults differ greatly from one another, as well as from their common
parent-form, the Gallus bankiva; and these characters are faithfully
transmitted by each breed to their offspring at the corresponding periods of
life. For instance, the chickens of spangled Hamburgs, whilst covered with
down, have a few dark spots on the head and rump, but are not striped
longitudinally, as in many other breeds; in their first true plumage,
“they are beautifully pencilled,” that is each feather is
transversely marked by numerous dark bars; but in their second plumage the
feathers all become spangled or tipped with a dark round spot. (34. These facts
are given on the high authority of a great breeder, Mr. Teebay; see
Tegetmeier’s ‘Poultry Book,’ 1868, p. 158. On the characters
of chickens of different breeds, and on the breeds of the pigeon, alluded to in
the following paragraph, see ‘Variation of Animals,’ etc., vol. i.
pp. 160, 249; vol. ii. p. 77.) Hence in this breed variations have occurred at,
and been transmitted to, three distinct periods of life. The Pigeon offers a
more remarkable case, because the aboriginal parent species does not undergo
any change of plumage with advancing age, excepting that at maturity the breast
becomes more iridescent; yet there are breeds which do not acquire their
characteristic colours until they have moulted two, three, or four times; and
these modifications of plumage are regularly transmitted.</p>
<h3>INHERITANCE AT CORRESPONDING SEASONS OF THE YEAR.</h3>
<p>With animals in a state of nature, innumerable instances occur of characters
appearing periodically at different seasons. We see this in the horns of the
stag, and in the fur of Arctic animals which becomes thick and white during the
winter. Many birds acquire bright colours and other decorations during the
breeding-season alone. Pallas states (35. ‘Novae species Quadrupedum e
Glirium ordine,’ 1778, p. 7. On the transmission of colour by the horse,
see ‘Variation of Animals and Plants under Domestication,’ vol. i.
p. 51. Also vol. ii. p. 71, for a general discussion on ‘Inheritance as
limited by Sex.’), that in Siberia domestic cattle and horses become
lighter-coloured during the winter; and I have myself observed, and heard of
similar strongly marked changes of colour, that is, from brownish cream-colour
or reddish-brown to a perfect white, in several ponies in England. Although I
do not know that this tendency to change the colour of the coat during
different seasons is transmitted, yet it probably is so, as all shades of
colour are strongly inherited by the horse. Nor is this form of inheritance, as
limited by the seasons, more remarkable than its limitation by age or sex.</p>
<p>INHERITANCE AS LIMITED BY SEX. — The equal transmission of characters to
both sexes is the commonest form of inheritance, at least with those animals
which do not present strongly-marked sexual differences, and indeed with many
of these. But characters are somewhat commonly transferred exclusively to that
sex, in which they first appear. Ample evidence on this head has been advanced
in my work on ‘Variation under Domestication,’ but a few instances
may here be given. There are breeds of the sheep and goat, in which the horns
of the male differ greatly in shape from those of the female; and these
differences, acquired under domestication, are regularly transmitted to the
same sex. As a rule, it is the females alone in cats which are tortoise-shell,
the corresponding colour in the males being rusty-red. With most breeds of the
fowl, the characters proper to each sex are transmitted to the same sex alone.
So general is this form of transmission that it is an anomaly when variations
in certain breeds are transmitted equally to both sexes. There are also certain
sub-breeds of the fowl in which the males can hardly be distinguished from one
another, whilst the females differ considerably in colour. The sexes of the
pigeon in the parent-species do not differ in any external character;
nevertheless, in certain domesticated breeds the male is coloured differently
from the female. (36. Dr. Chapuis, ‘Le Pigeon Voyageur Belge,’
1865, p. 87. Boitard et Corbie, ‘Les Pigeons de Volière,’ etc.,
1824, p. 173. See, also, on similar differences in certain breeds at Modena,
‘Le variazioni dei Colombi domestici,’ del Paolo Bonizzi, 1873.)
The wattle in the English Carrier pigeon, and the crop in the Pouter, are more
highly developed in the male than in the female; and although these characters
have been gained through long-continued selection by man, the slight
differences between the sexes are wholly due to the form of inheritance which
has prevailed; for they have arisen, not from, but rather in opposition to, the
wish of the breeder.</p>
<p>Most of our domestic races have been formed by the accumulation of many slight
variations; and as some of the successive steps have been transmitted to one
sex alone, and some to both sexes, we find in the different breeds of the same
species all gradations between great sexual dissimilarity and complete
similarity. Instances have already been given with the breeds of the fowl and
pigeon, and under nature analogous cases are common. With animals under
domestication, but whether in nature I will not venture to say, one sex may
lose characters proper to it, and may thus come somewhat to resemble the
opposite sex; for instance, the males of some breeds of the fowl have lost
their masculine tail-plumes and hackles. On the other hand, the differences
between the sexes may be increased under domestication, as with merino sheep,
in which the ewes have lost their horns. Again, characters proper to one sex
may suddenly appear in the other sex; as in those sub-breeds of the fowl in
which the hens acquire spurs whilst young; or, as in certain Polish sub-breeds,
in which the females, as there is reason to believe, originally acquired a
crest, and subsequently transferred it to the males. All these cases are
intelligible on the hypothesis of pangenesis; for they depend on the gemmules
of certain parts, although present in both sexes, becoming, through the
influence of domestication, either dormant or developed in either sex.</p>
<p>There is one difficult question which it will be convenient to defer to a
future chapter; namely, whether a character at first developed in both sexes,
could through selection be limited in its development to one sex alone. If, for
instance, a breeder observed that some of his pigeons (of which the characters
are usually transferred in an equal degree to both sexes) varied into pale
blue, could he by long-continued selection make a breed, in which the males
alone should be of this tint, whilst the females remained unchanged? I will
here only say, that this, though perhaps not impossible, would be extremely
difficult; for the natural result of breeding from the pale-blue males would be
to change the whole stock of both sexes to this tint. If, however, variations
of the desired tint appeared, which were from the first limited in their
development to the male sex, there would not be the least difficulty in making
a breed with the two sexes of a different colour, as indeed has been effected
with a Belgian breed, in which the males alone are streaked with black. In a
similar manner, if any variation appeared in a female pigeon, which was from
the first sexually limited in its development to the females, it would be easy
to make a breed with the females alone thus characterised; but if the variation
was not thus originally limited, the process would be extremely difficult,
perhaps impossible. (37. Since the publication of the first edition of this
work, it has been highly satisfactory to me to find the following remarks (the
‘Field,’ Sept. 1872) from so experienced a breeder as Mr.
Tegetmeier. After describing some curious cases in pigeons, of the transmission
of colour by one sex alone, and the formation of a sub-breed with this
character, he says: “It is a singular circumstance that Mr. Darwin should
have suggested the possibility of modifying the sexual colours of birds by a
course of artificial selection. When he did so, he was in ignorance of these
facts that I have related; but it is remarkable how very closely he suggested
the right method of procedure.”)</p>
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