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$Pretitle{}
$Title{Descent Of Man, The
Chapter 8.2}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{male
males
sexes
female
sex
characters
females
animals
selection
alone}
$Date{1874}
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Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 8.2

     The Male Generally More Modified than the Female. - Throughout the animal
kingdom when the sexes differ in external appearance, it is, with rare
exceptions, the male which has been the more modified; for, generally, the
female retains a closer resemblance to the young of her own species and to
other adult members of the same group.  The cause of this seems to lie in the
males of almost all animals having stronger passions than the females.  Hence
it is the males that fight together and sedulously display their charms before
the females; and the victors transmit their superiority to their male
offspring.  Why both sexes do not thus acquire the characters of their fathers
will be considered hereafter.  That the males of all mammals eagerly pursue
the females is notorious to every one.  So it is with birds; but many cock
birds do not so much pursue the hen, as display their plumage, perform strange
antics, and pour forth their song in her presence.  The male in the few fish
observed seems much more eager than the female; and the same is true of
alligators, and apparently of Batrachians.  Throughout the enormous class of
insects, as Kirby remarks, ^450 "the law is that the male shall seek the
female." Two good authorities, Mr. Blackwall and Mr. D. C. Spence Bate, tell
me that the males of spiders and crustaceans are more active and more erratic
in their habits than the females.  When the organs of sense or locomotion are
present in the one sex of insects and crustaceans and absent in the other, or
when, as is frequently the case, they are more highly developed in the one
than in the other, it is, as far as I can discover, almost invariably the male
which retains such organs, or has them most developed; and this shows that the
male is the more active member in the courtship of the sexes. ^451

[Footnote 450: Kirby and Spence, "Introduction to Entomology," vol. iii, 1826,
p. 342.]

[Footnote 451: One parasitic Hymenopterous insect (Westwood, "Modern Class. of
Insects," vol. ii, p. 160) forms an exception to the rule, as the male has
rudimentary wings, and never quits the cell in which it is born, while the
female has well-developed wings.  Audouin believes that the females of this
species are impregnated by the males which are born in the same cells with
them; but it is much more probable that the females visit other cells, so that
close interbreeding is thus avoided. We shall hereafter meet in various
classes, with a few exceptional cases, in which the female, instead of the
male, is the seeker and wooer.]

     The female, on the other hand, with the rarest exceptions, is less eager
than the male.  As the illustrious Hunter ^452 long ago observed she generally
"requires to be courted;" she is coy, and may often be seen endeavoring for a
long time to escape from the male.  Every observer of the habits of animals
will be able to call to mind instances of this kind.  It is shown by various
facts, given hereafter, and by the results fairly attributable to sexual
selection, that the female, though comparatively passive, generally exerts
some choice and accepts one male in preference to others.  Or she may accept,
as appearances would sometimes lead us to believe, not the male which is the
most attractive to her, but the one which is the least distasteful.  The
exertion of some choice on the part of the female seems a law almost as
general as the eagerness of the male.

[Footnote 452: "Essays and Observations," edited by Owen, vol. i, 1861, p.
194.]

     We are naturally led to inquire why the male, in so many and such
distinct classes, has become more eager than the female, so that he searches
for her and plays the more active part in courtship.  It would be no advantage
and some loss of power if each sex searched for the other; but why should the
male almost always be the seeker?  The ovules of plants after fertilization
have to be nourished for a time; hence the pollen is necessarily brought to
the female organs - being placed on the stigma, by means of insects or the
wind, or by the spontaneous movements of the stamens; and in the Algae, etc.,
by the locomotive power of the antherozooids.  With lowly-organized aquatic
animals, permanently affixed to the same spot and having their sexes separate,
the male element is invariably brought to the female; and of this we can see
the reason, for even if the ova were detached before fertilization, and did
not require subsequent nourishment or protection, there would yet be greater
difficulty in transporting them than the male element, because, being larger
than the latter, they are produced in far smaller numbers.  So that many of
the lower animals are, in this respect, analogous with plants. ^453 The males
of affixed and aquatic animals having been led to emit their fertilizing
elements in this way, it is natural that any of their descendants, which rose
in the scale and became locomotive, should retain the same habit; and they
would approach the female as closely as possible, in order not to risk the
loss of the fertilizing element in a long passage of it through the water.
With some few of the lower animals, the females alone are fixed, and the males
of these must be the seekers.  But it is difficult to understand why the males
of species, of which the progenitors were primordially free, should invariably
have acquired the habit of approaching the females, instead of being
approached by them.  But in all cases, in order that the males should seek
efficiently, it would be necessary that they should be endowed with strong
passions; and the acquirement of such passions would naturally follow from the
more eager leaving a larger number of offspring than the less eager.

[Footnote 453: Prof. Sachs ("Lehrbuch der Botanik," 1870, s. 633), in speaking
of the male and female reproductive cells, remarks, "verhalt sich die eine bei
der Vereinigung activ, . . . die andere erscheint bei der Vereinigung
passiv."]

     The great eagerness of the males has thus indirectly led to their much
more frequently developing secondary sexual characters than the females.  But
the development of such characters would be much aided if the males were more
liable to vary than the females - as I concluded they were - after a long
study of domesticated animals.  Von Nathusius, who has had very wide
experience, is strongly of the same opinion. ^454 Good evidence also in favor
of this conclusion can be produced by a comparison of the two sexes in
mankind.  During the Novara Expedition ^455 a vast number of measurements was
made of various parts of the body in different races, and the men were found
in almost every case to present a greater range of variation than the women;
but I shall have to recur to this subject in a future chapter.  Mr. J. Wood,
^456 who has carefully attended to the variation of the muscles in man, puts
in italics the conclusion that "the greatest number of abnormalities in each
subject is found in the males." He had previously remarked that "altogether in
102 subjects, the varieties of redundancy were found to be half as many again
as in females, contrasting widely with the greater frequency of deficiency in
females before described." Prof. Macalister likewise remarks ^457 that
variations in the muscles "are probably more common in males than females."
Certain muscles which are not normally present in mankind are also more
frequently developed in the male than in the female sex, although exceptions
to this rule are said to occur.  Dr. Burt Wilder ^458 has tabulated the cases
of 152 individuals with supernumerary digits, of which 86 were males and 39,
or less than half, females, the remaining 27 being of unknown sex.  It should
not, however, be overlooked that women would more frequently endeavor to
conceal a deformity of this kind than men.  Again, Dr. L. Meyer asserts that
the ears of man are more variable in form than those of a woman. ^459 Lastly,
the temperature is more variable in man than in woman. ^460

[Footnote 454: "Vortrage uber Viehzucht," 1872, p. 63.]

[Footnote 455: "Reise der Novara; Anthropolog. Theil," 1867, ss. 216-269. The
results were calculated by Dr. Weisbach from measurements made by Drs. K.
Scherzer and Schwarz.  On the greater variability of the males of domesticated
animals, see my "Variation of Animals and Plants under Domestication," vol.
ii, 1868, p. 75.]

[Footnote 456: "Proceedings Royal Soc.," vol. xvi, July, 1868, pp. 519, 524.]

[Footnote 457: "Proc. Royal Irish Academy," vol. x, 1868, p. 123.]

[Footnote 458: "Massachusetts Medical Soc.," vol. ii, No. 3, 1868, p. 9.]

[Footnote 459: "Archiv fur Path. Anat. und Phys.," 1871, p. 488.]

[Footnote 460: The conclusions recently arrived at by Dr. J. Stockton Hough,
on the temperature of man, are given in the "Pop. Science Review," Jan. 1,
1874, p. 97.]

     The cause of the greater general variability in the male sex than in the
female is unknown, except in so far as secondary sexual characters are
extraordinarily variable and are usually confined to the males; and, as we
shall presently see, this fact is, to a certain extent, intelligible.  Through
the action of sexual and natural selection male animals have been rendered in
very many instances widely different from their females; but independently of
selection the two sexes, from differing constitutionally, tend to vary in a
somewhat different manner. The female has to expend much organic matter in the
formation of her ova, whereas the male expends much force in fierce contests
with his rivals, in wandering about in search of the female, in exerting his
voice, pouring out odoriferous secretions, etc.; and this expenditure is
generally concentrated within a short period.  The great vigor of the male
during the season of love seems often to intensify his colors independently of
any marked difference from the female. ^461 In mankind, and even as low down
in the organic scale as in the Lepidoptera, the temperature of the body is
higher in the male than in the female, accompanied in the case of man by a
slower pulse. ^462 On the whole, the expenditure of matter and force by the
two sexes is probably nearly equal, though effected in very different ways and
at different rates.

[Footnote 461: Prof. Mantegazza is inclined to believe ("Lettera a Carlo
Darwin," "Archivio per l'Anthropologia," 1871, p. 306) that the bright colors,
common in so many male animals, are due to the presence and retention by them
of the spermatic fluid; but this can hardly be the case; for many male birds,
for instance young pheasants, become brightly colored in the autumn of their
first year.]

[Footnote 462: For mankind, see Dr. J. Stockton Hough, whose conclusions are
given in the "Pop. Science Review," 1874, p. 97.  See Girard's observations on
the Lepidoptera, as given in the "Zoological Record," 1869, p. 347.]

     From the causes just specified the two sexes can hardly fail to differ
somewhat in constitution, at least during the breeding season; and although
they may be subjected to exactly the same conditions they will tend to vary in
a different manner.  If such variations are of no service to either sex they
will not be accumulated and increased by sexual or natural selection.
Nevertheless, they may become permanent if the exciting cause acts
permanently; and in accordance with a frequent form of inheritance they may be
transmitted to that sex alone in which they first appeared.  In this case the
two sexes will come to present permanent, yet unimportant, differences of
character.  For instance, Mr. Allen shows that with a large number of birds
inhabiting the northern and southern United States, the specimens from the
south are darker-colored than those from the north; and this seems to be the
direct result of the difference in temperature, light, etc., between the two
regions.  Now, in some few cases, the two sexes of the same species appear to
have been differently affected; in the Ageloeus phoeniceus the males have had
their colors greatly intensified in the south; whereas with Cardinalis
virginianus it is the females which have been thus affected; with Quiscalus
major the females have been rendered extremely variable in tint, while the
males remain nearly uniform. ^463

[Footnote 463: "Mammals and Birds of E. Florida," pp. 234, 280, 295.]

     A few exceptional cases occur in various classes of animals, in which the
females instead of the males have acquired well-pronounced secondary sexual
characters, such as brighter colors, greater size, strength or pugnacity.
With birds there has sometimes been a complete transposition of the ordinary
characters proper to each sex; the females having become the more eager in
courtship, the males remaining comparatively passive, but apparently selecting
the more attractive females, as we may infer from the results.  Certain hen
birds have thus been rendered more highly colored or otherwise ornamented, as
well as more powerful and pugnacious than the cocks; these characters being
transmitted to the female offspring alone.

     It may be suggested that in some cases a double process of selection has
been carried on; that the males have selected the more attractive females and
the latter the more attractive males.  This, process, however, though it might
lead to the modification of both sexes, would not make the one sex different
from the other, unless indeed their tastes for the beautiful differed; but
this is a supposition too improbable to be worth considering in the case of
any animal, excepting man.  There are, however, many animals in which the
sexes resemble each other, both being furnished with the same ornaments, which
analogy would lead us to attribute to the agency of sexual selection.  In such
cases it may be suggested with more plausibility that there has been a double
or mutual process of sexual selection; the more vigorous and precocious
females selecting the more attractive and vigorous males, the latter rejecting
all except the more attractive females.  But from what we know of the habits
of animals, this view is hardly probable, for the male is generally eager to
pair with any female.  It is more probable that the ornaments common to both
sexes were acquired by one sex, generally the male, and then transmitted to
the offspring of both sexes.  If, indeed, during a lengthened period the males
of any species were greatly to exceed the females in number, and then during
another lengthened period, but under different conditions, the reverse were to
occur, a double, but not simultaneous, process of sexual selection might
easily be carried on, by which the two sexes might be rendered widely
different.

     We shall hereafter see that many animals exist, of which neither sex is
brilliantly colored or provided with special ornaments, and yet the members of
both sexes or of one alone have probably acquired simple colors, such as white
or black, through sexual selection.  The absence of bright tints or other
ornaments may be the result of variations of the right kind never having
occurred, or of the animals themselves having preferred plain black or white.
Obscure tints have often been developed through natural selection for the sake
of protection, and the acquirement through sexual selection of conspicuous
colors appears to have been sometimes checked from the danger thus incurred.
But in other cases the males during long ages may have struggled together for
the possession of the females, and yet no effect will have been produced,
unless a larger number of offspring were left by the more successful males to
inherit their superiority than by the less successful; and this, as previously
shown, depends on many complex contingencies.

     Sexual selection acts in a less rigorous manner than natural selection.
The latter produces its effects by the life or death at all ages of the more
or less successful individuals.  Death, indeed, not rarely ensues from the
conflicts of rival males.  But generally the less successful male merely fails
to obtain a female, or obtains a retarded and less vigorous female later in
the season, or, if polygamous, obtains fewer females; so that they leave
fewer, less vigorous, or no offspring. In regard to structures acquired
through ordinary or natural selection there is in most cases, as long as the
conditions of life remain the same, a limit to the amount of advantageous
modification in relation to certain special purposes; but in regard to
structures adapted to make one male victorious over another, either in
fighting or in charming the female, there is no definite limit to the amount
of advantageous modification; so that as long as the proper variations arise
the work of sexual selection will go on.  This circumstance may partly account
for the frequent and extraordinary amount of variability presented by
secondary sexual characters.  Nevertheless, natural selection will determine
that such characters shall not be acquired by the victorious males, if they
would be highly injurious, either by expending too much of their vital powers
or by exposing them to any great danger.  The development, however, of certain
structures - of the horns, for instance, in certain stags - has been carried
to a wonderful extreme; and in some cases to an extreme which, as far as the
general conditions of life are concerned, must be slightly injurious to the
male.  From this fact we learn that the advantages which favored males derive
from conquering other males in battle or courtship, and thus leaving a
numerous progeny, are in the long run greater than those derived from rather
more perfect adaptation to their conditions of life.  We shall further see,
and it could never have been anticipated, that the power to charm the female
has sometimes been more important than the power to conquer other males in
battle.

     Laws of Inheritance. - 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 evidence 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. ^464 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 suckled their young 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 favorable 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.

[Footnote 464: H. Muller, "Anwendung der Darwin'schen Lehre," etc.  Verh. d.
n. V. Jahrg. xxix, p. 42.]

     Inheritance at Corresponding Periods of Life. - 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, ^465 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, while 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 penciled," 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. ^466 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 at
maturity the breast becomes more iridescent; yet there are breeds which do not
acquire their characteristic colors until they have moulted two, three, or
four times; and these modifications of plumage are regularly transmitted.

[Footnote 465: "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.]

[Footnote 466: 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.]

     Inheritance at Corresponding Seasons of the Year. - 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 the Arctic animals, which becomes thick and white during the
winter.  Many birds acquire bright colors and other decorations during the
breeding-season alone.  Pallas states, ^467 that in Siberia domestic cattle
and horses become lighter-colored during the winter; and I have myself
observed, and heard of similar strongly-marked changes of color, that is, from
brownish-cream color or reddish-brown to a perfect white, in several ponies in
England.  Although I do not know that this tendency to change the color of the
coat during different seasons is transmitted, yet it probably is so, as all
shades of color 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.

[Footnote 467: "Novae species Quadrupedum e Glirium ordine," 1778, p. 7. On
the transmission of color by the horse, see "Variation of Animals, etc., under
Domestication," vol. i, p. 51.  Also vol. ii, p. 71, for a general discussion
on "Inheritance as Limited by Sex."]

     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 color 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, while the females differ considerably in color.  The sexes
of the pigeon in the parent-species do not differ in any external character;
nevertheless, in certain domesticated breeds the male is colored differently
from the female. ^468 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.

[Footnote 468: Dr. Chapuis, "Le Pigeon Voyageur Belge," 1865, p. 87. Boitard
et Corbie, "Les Pigeons de Voliere," etc., 1824, p. 173.  See, also, on
similar differences in certain breeds at Modena, "Le variazioni dei Colombi
domestici," del Paolo Bonizzi, 1873.]

     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 while 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.

     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, while 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 color, 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
characterized; but if the variation was not thus originally limited the
process would be extremely difficult, perhaps impossible. ^469

[Footnote 469: 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 color 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 colors 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."]

     On the Relation Between the Period of Development of a Character and Its
Transmission to One Sex or to Both Sexes. - Why certain characters should be
inherited by both sexes and other characters by one sex alone, namely, by that
sex in which the character first appeared, is in most cases quite unknown.  We
cannot even conjecture why with certain sub-breeds of the pigeon black striae,
though transmitted through the female, should be developed in the male alone,
while every other character is equally transferred to both sexes.  Why, again,
with cats, the tortoise-shell color should, with rare exceptions, be developed
in the female alone.  The very same character, such as deficient or
supernumerary digits, color-blindness, etc., may with mankind be inherited by
the males alone of one family, and in another family by the females alone,
though in both cases transmitted through the opposite as well as through the
same sex. ^470 Although we are thus ignorant, the two following rules seem
often to hold good - that variations which first appear in either sex at a
late period of life tend to be developed in the same sex alone; while
variations which first appear early in life in either sex tend to be developed
in both sexes.  I am, however, far from supposing that this is the sole
determining cause.  As I have not elsewhere discussed this subject, and as it
has an important bearing on sexual selection, I must here enter into lengthy
and somewhat intricate details.

[Footnote 470: References are given in my "Variation of Animals under
Domestication," vol. ii, p. 72.]

     It is in itself probable that any character appearing at an early age
would tend to be inherited equally by both sexes, for the sexes do not differ
much in constitution before the power of reproduction is gained.  On the other
hand, after this power has been gained and the sexes have come to differ in
constitution, the gemmules (if I may again use the language of pangenesis)
which are cast off from each varying part in the one sex would be much more
likely to possess the proper affinities for uniting with the tissues of the
same sex and thus becoming developed than with those of the opposite sex.

     I was first led to infer that a relation of this kind exists from the
fact that whenever and in whatever manner the adult male differs from the
adult female, he differs in the same manner from the young of both sexes.  The
generality of this fact is quite remarkable; it holds good with almost all
mammals, birds, amphibians and fishes; also with many crustaceans, spiders and
some few insects, such as certain orthoptera and libellulae.  In all these
cases the variations, through the accumulation of which the male acquired his
proper masculine characters, must have occurred at a somewhat late period of
life; otherwise the young males would have been similarly characterized; and
conformably with our rule, the variations are transmitted to and developed in
the adult males alone. When, on the other hand, the adult male closely
resembles the young of both sexes (these, with rare exceptions, being alike),
he generally resembles the adult female; and in most of these cases the
variations through which the young and old acquired their present characters,
probably occurred, according to our rule, during youth.  But there is here
room for doubt, for characters are sometimes transferred to the offspring at
an earlier age than that at which they first appeared in the parents, so that
the parents may have varied when adult and have transferred their characters
to their offspring while young.  There are, moreover, many animals in which
the two sexes closely resemble each other, and yet both differ from their
young; and here the characters of the adults must have been acquired late in
life; nevertheless, these characters, in apparent contradiction to our rule,
are transferred to both sexes.  We must not, however, overlook the possibility
or even probability of successive variations of the same nature occurring,
under exposure to similar conditions, simultaneously in both sexes at a rather
late period of life; and in this case the variations would be transferred to
the offspring of both sexes at a corresponding late age; and there would then
be no real contradiction to the rule that variations occurring late in life
are transferred exclusively to the sex in which they first appeared.  This
latter rule seems to hold true more generally than the second one, namely,
that variations which occur in either sex early in life tend to be transferred
to both sexes.  As it was obviously impossible even to estimate in how large a
number of cases throughout the animal kingdom these two propositions held
good, it occurred to me to investigate some striking or crucial instances and
to rely on the result.