History of the World/History of the World (Bureau Development, Inc.)(1992).BIN/dp/0107/01078.txt

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$Unique_ID{how01078}
$Pretitle{}
$Title{Descent Of Man, The
Chapter 8.3}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{sexes
male
males
life
female
horns
species
characters
period
sex}
$Date{1874}
$Log{}
Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 8.3

     An excellent case for investigation is afforded by the deer family. In
all the species but one the horns are developed only in the males, though
certainly transmitted through the females and capable of abnormal development
in them.  In the reindeer, on the other hand, the female is provided with
horns; so that in this species the horns ought, according to our rule, to
appear early in life, long before the two sexes are mature and have come to
differ much in constitution.  In all the other species the horns ought to
appear later in life, which would lead to their development in that sex alone
in which they first appeared in the progenitor of the whole family.  Now in
seven species belonging to distinct sections of the family and inhabiting
different regions in which the stags alone bear horns, I find that the horns
first appear at periods varying from nine months after birth in the roebuck to
ten, twelve or even more months in the stags of the six other and larger
species. ^471 But with the reindeer the case is widely different; for, as I
hear from Prof. Nilsson, who kindly made special inquiries for me in Lapland,
the horns appear in the young animals within four or five weeks after birth,
and at the same time in both sexes.  So that here we have a structure
developed at a most unusually early age in one species of the family and
likewise common to both sexes in this one species alone.

[Footnote 471: I am much obliged to Mr. Cupples for having made inquiries for
me in regard to the Roebuck and Red Deer of Scotland from Mr. Robertson, the
experienced head-forester to the Marquis of Breadalbane. In regard to
Fallow-deer, I have to thank Mr. Eyton and others for information.  For the
Cervus alces of North America, see "Land and Water," 1868, pp. 221, 254; and
for the C. Virginianus and strongyloceros of the same continent, see J. D.
Caton, in "Ottawa Acad. of Nat. Sc.," 1868, p. 13.  For Cervus Eldi of Pegu,
see Lieut.  Beavan, "Proc. Zoolog. Soc.," 1867, p. 762.]

     In several kinds of antelopes only the males are provided with horns,
while in the greater number both sexes bear horns.  With respect to the period
of development, Mr. Blyth informs me that there was at one time in the
Zoological Gardens a young koodoo (Ant. strepsiceros) of which the males alone
are horned, and also the young of a closely allied species, the eland (Ant.
oreas), in which both sexes are horned.  Now it is in strict conformity with
our rule that in the young male koodoo, although ten months old, the horns
were remarkably small, considering the size ultimately attained by them; while
in the young male eland, although only three months old, the horns were
already very much larger than in the koodoo.  It is also a noticeable fact
that in the prong-horned antelope ^472 only a few of the females, about one in
five, have horns, and these are in a rudimentary state, though sometimes above
four inches long; so that as far as concerns the possession of horns by the
males alone, this species is in an intermediate condition and the horns do not
appear until about five or six months after birth.  Therefore in comparison
with what little we know of the development of the horns in other antelopes
and from what we do know with respect to the horns of deer, cattle, etc.,
those of the prong-horned antelope appear at an intermediate period of life -
that is not very early, as in cattle and sheep, nor very late, as in the
larger deer and antelopes.  The horns of sheep, goats and cattle which are
well developed in both sexes, though not quite equal in size, can be felt, or
even seen, at birth or soon afterward. ^473 Our rule, however, seems to fail
in some breeds of sheep, for instance merinos, in which the rams alone are
horned; for I cannot find on inquiry ^474 that the horns are developed later
in life in this breed than in ordinary sheep in which both sexes are horned.
But with domesticated sheep the presence or absence of horns is not a firmly
fixed character; for a certain proportion of the merino ewes bear small horns,
and some of the rams are hornless; and in most breeds hornless ewes are
occasionally produced.

[Footnote 472: Antilocapra Americana.  I have to thank Dr. Canfield for
information with respect to the horns of the female; see also his paper in
"Proc. Zoolog. Soc.," 1866, p. 109.  Also Owen, "Anatomy of Vertebrates," vol.
iii, p. 627.]

[Footnote 473: I have been assured that the horns of the sheep in North Wales
can always be felt, and are sometimes even an inch in length at birth.  Youatt
says ("Cattle," 1834, p. 277), that the prominence of the frontal bone in
cattle penetrates the cutis at birth, and that the horny matter is soon formed
over it.]

[Footnote 474: I am greatly indebted to Prof. Victor Carus for having made
inquiries for me, from the highest authorities, with respect to the merino
sheep of Saxony.  On the Guinea coast of Africa there is, however, a breed of
sheep in which, as with merinos, the rams alone bear horns; and Mr. Winwood
Reade informs me that in one case observed by him, a young ram, born on Feb.
10th, first showed horns on March 6th, so that in this instance, in conformity
with rule, the development of the horns occurred at a later period of life
than in Welsh sheep, in which both sexes are horned.]

     Dr. W. Marshall has lately made a special study of the protuberances so
common on the heads of birds, ^475 and he comes to the following conclusion:
that with those species in which they are confined to the males, they are
developed late in life; whereas with those species in which they are common to
the two sexes, they are developed at a very early period.  This is certainly a
striking confirmation of my two laws of inheritance.

[Footnote 475: "Ueber die knochermen Schadelhocker der Vogel" in the
"Niederlandischen Archiv. fur Zoologie," Band I, Heft 2, 1872.]

     In most of the species of the splendid family of the pheasants, the males
differ conspicuously from the females, and they acquire their ornaments at a
rather late period of life.  The eared pheasant (Crossoptilon auritum),
however, offers a remarkable exception, for both sexes possess the fine caudal
plumes, the large ear-tufts and the crimson velvet about the head; I find that
all these characters appear very early in life in accordance with rule.  The
adult male can, however, be distinguished from the adult female by the
presence of spurs; and, conformably with our rule, these do not begin to be
developed before the age of six months, as I am assured by Mr. Bartlett, and
even at this age the two sexes can hardly be distinguished. ^476 The male and
female peacock differ conspicuously from each other in almost every part of
their plumage, except in the elegant head-crest, which is common to both
sexes; and this is developed very early in life, long before the other
ornaments which are confined to the male.  The wild duck offers an analogous
case, for the beautiful green speculum on the wings is common to both sexes,
though duller and somewhat smaller in the female, and it is developed early in
life, while the curled tail-feathers and other ornaments of the male are
developed later. ^477 Between such extreme cases of close sexual resemblance
and wide dissimilarity, as those of the Crossoptilon and peacock, many
intermediate ones could be given, in which the characters follow our two rules
in their order of development.

[Footnote 476: In the common peacock (Pavo cristatus) the male alone possesses
spurs, while both sexes of the Java Peacock (P. muticus) offer the unusual
case of being furnished with spurs.  Hence I fully expected that in the latter
species they would have been developed earlier in life than in the common
peacock; but M. Hegt of Amsterdam informs me, that with young birds of the
previous year, of both species, compared on April 23, 1869, there was no
difference in the development of the spurs.  The spurs, however, were as yet
represented merely by slight knobs or elevations.  I presume that I should
have been informed if any difference in the rate of development had been
observed subsequently.]

[Footnote 477: In some other species of the Duck family the speculum differs
in a greater degree in the two sexes; but I have not been able to discover
whether its full development occurs later in life in the males of such
species, than in the males of the common duck, as ought to be the case
according to our rule.  With the allied Mergus cucullatus we have, however, a
case of this kind: the two sexes differ conspicuously in general plumage, and
to a considerable degree in the speculum, which is pure white in the male and
grayish white in the female.  Now the young males at first entirely resemble
the females, and have a grayish-white speculum, which becomes pure white at an
earlier age than that at which the adult male acquires his other and more
strongly-marked sexual differences: see Audubon, "Ornithological Biography,"
vol. iii, 1835, pp. 249-250.]

     As most insects emerge from the pupal state in a mature condition it is
doubtful whether the period of development can determine the transference of
their characters to one or to both sexes.  But we do not know that the colored
scales, for instance, in two species of butterflies, in one of which the sexes
differ in color, while in the other they are alike, are developed at the same
relative age in the cocoon.  Nor do we know whether all the scales are
simultaneously developed on the wings of the same species of butterfly, in
which certain colored marks are confined to one sex, while others are common
to both sexes.  A difference of this kind in the period of development is not
so improbable as it may at first appear; for with the Orthoptera, which assume
their adult state, not by a single metamorphosis, but by a succession of
moults, the young males of some species at first resemble the females, and
acquire their distinctive masculine characters only at a later moult.
Strictly analogous cases occur at the successive moults of certain male
crustaceans.

     We have as yet considered the transference of characters, relatively to
their period of development, only in species in a natural state; we will now
turn to domesticated animals, and first touch on monstrosities and diseases.
The presence of supernumerary digits, and the absence of certain phalanges,
must be determined at an early embryonic period - the tendency to profuse
bleeding is at least congenital, as is probably color-blindness - yet these
peculiarities, and other similar ones, are often limited in their transmission
to one sex; so that the rule that characters, developed at an early period,
tend to be transmitted to both sexes, here wholly fails.  But this rule, as
before remarked, does not appear to be nearly so general as the converse one,
namely, that characters which appear late in life in one sex are transmitted
exclusively to the same sex.  From the fact of the above abnormal
peculiarities becoming attached to one sex, long before the sexual functions
are active, we may infer that there must be some difference between the sexes
at an extremely early age.  With respect to sexually-limited diseases we know
too little of the period at which they originate to draw any safe conclusion.
Gout, however, seems to fall under our rule, for it is generally caused by
intemperance during manhood, and is transmitted from the father to his sons in
a much more marked manner than to his daughters.

     In the various domestic breeds of sheep, goats, and cattle the males
differ from their respective females in the shape or development of their
horns, forehead, mane, dewlap, tail and hump on the shoulders; and these
peculiarities, in accordance with our rule, are not fully developed until a
rather late period of life.  The sexes of dogs do not differ, except that in
certain breeds, especially in the Scotch deer-hound, the male is much larger
and heavier than the female; and, as we shall see in a future chapter, the
male goes on increasing in size to an unusually late period of life, which,
according to rule, will account for his increased size being transmitted to
his male offspring alone.  On the other hand, the tortoise-shell color, which
is confined to female cats, is quite distinct at birth, and this case violates
the rule.  There is a breed of pigeons in which the males alone are streaked
with black, and the streaks can be detected even in the nestlings; but they
become more conspicuous at each successive moult, so that this case partly
opposes and partly supports the rule.  With the English carrier and Pouter
pigeons, the full development of the wattle and crop occurs rather late in
life, and conformably with the rule, these characters are transmitted in full
perfection to the males alone.  The following cases perhaps come within the
class previously alluded to, in which both sexes have varied in the same
manner at a rather late period of life, and have consequently transferred
their new characters to both sexes at a corresponding late period; and if so,
these cases are not opposed to our rule; there exist sub-breeds of the pigeon,
described by Neumeister, ^478 in which both sexes change their color during
two or three moults (as is likewise the case with the Almond Tumbler);
nevertheless, these changes, though occurring rather late in life, are common
to both sexes.  One variety of the canary-bird, namely, the London Prize,
offers a nearly analogous case.

[Footnote 478: "Das Ganze der Taubenzucht," 1837, ss. 21, 24.  For the case of
the streaked pigeons, see Dr. Chapuis, "Le pigeon voyageur Belge," 1865, p.
87.]

     With the breeds of the fowl the inheritance of various characters by one
or both sexes seems generally determined by the period at which such
characters are developed.  Thus in all the many breeds in which the adult male
differs greatly in color from the female, as well as from the wild
parent-species, he differs also from the young male, so that the
newly-acquired characters must have appeared at a rather late period of life.
On the other hand, in most of the breeds in which the two sexes resemble each
other, the young are colored in nearly the same manner as their parents, and
this renders it probable that their colors first appeared early in life.  We
have instances of this fact in all black and white breeds, in which the young
and old of both sexes are alike; nor can it be maintained that there is
something peculiar in a black or white plumage, which leads to its
transference to both sexes; for the males alone of many natural species are
either black or white, the females being differently colored.  With the
so-called Cuckoo sub-breeds of the fowl in which the feathers are transversely
penciled with dark stripes, both sexes and the chickens are colored in nearly
the same manner.  The laced plumage of the Sebright bantam is the same in both
sexes, and in the young chickens the wing-feathers are distinctly, though
imperfectly, laced.  Spangled Hamburgs, however, offer a partial exception;
for the two sexes, though not quite alike, resemble each other more closely
than do the sexes of the aboriginal parent-species; yet they acquire their
characteristic plumage late in life, for the chickens are distinctly penciled.
With respect to other characters besides color, in the wild-parent species and
in most of the domestic breeds the males alone possess a well-developed comb;
but in the young of the Spanish fowl it is largely developed at a very early
age, and, in accordance with this early development in the male, it is of
unusual size in the adult female.  In the game breeds pugnacity is developed
at a wonderfully early age, of which curious proofs could be given; and this
character is transmitted to both sexes, so that the hens, from their extreme
pugnacity, are now generally exhibited in separate pens.  With the Polish
breeds the bony protuberance of the skull which supports the crest is
partially developed even before the chickens are hatched, and the crest itself
soon begins to grow, though at first feebly; ^479 and in this breed the adults
of both sexes are characterized by a great bony protuberance and an immense
crest.

[Footnote 479: For full particulars and references on all these points
respecting the several breeds of the fowl, see "Variation of Animals and
Plants under Domestication," vol. i, pp. 250, 256.  In regard to the higher
animals, the sexual differences which have arisen under domestication are
described in the same work under the head of each species.]

     Finally, from what we have now seen of the relation which exists in many
natural species and domesticated races between the period of the development
of their characters and the manner of their transmission - for example, the
striking fact of the early growth of the horns in the reindeer, in which both
sexes bear horns, in comparison with their much later growth in the other
species in which the male alone bears horns - we may conclude that one, though
not the sole cause of characters being exclusively inherited by one sex, is
their development at a late age. And secondly, that one, though apparently a
less effective cause of characters being inherited by both sexes, is their
development at an early age, while the sexes differ but little in
constitution.  It appears, however, that some difference must exist between
the sexes even during a very early embryonic period, for characters developed
at this age not rarely become attached to one sex.

     Summary and Concluding Remarks.  - From the foregoing discussion on the
various laws of inheritance we learn that the characters of the parents often,
or even generally, tend to become developed in the offspring of the same sex,
at the same age, and periodically at the same season of the year in which they
first appeared in the parents.  But these rules, owing to unknown causes, are
far from being fixed.  Hence, during the modification of a species the
successive changes may readily be transmitted in different ways; some to one
sex and some to both; some to the offspring at one age and some to the
offspring at all ages.  Not only are the laws of inheritance extremely
complex, but so are the causes which induce and govern variability.  The
variations thus induced are preserved and accumulated by sexual selection,
which is in itself an extremely complex affair, depending as it does on the
ardor of love, the courage and the rivalry of the males as well as on the
powers of perception, the taste and will of the female.  Sexual selection will
also be largely dominated by natural selection tending toward the general
welfare of the species.  Hence the manner in which the individuals of either
or both sexes have been affected through sexual selection cannot fail to be
complex in the highest degree.

     When variations occur late in life in one sex and are transmitted to the
same sex at the same age the other sex and the young are left unmodified.
When they occur late in life but are transmitted to both sexes at the same age
the young alone are left unmodified.  Variations, however, may occur at any
period of life in one sex or in both, and be transmitted to both sexes at all
ages, and then all the individuals of the species are similarly modified.  In
the following chapters it will be seen that all these cases frequently occur
in nature.

     Sexual selection can never act on any animal before the age for
reproduction arrives.  From the great eagerness of the male it has generally
acted on this sex and not on the females.  The males have thus become provided
with weapons for fighting with their rivals, with organs for discovering and
securely holding the female and for exciting or charming her.  When the sexes
differ in these respects it is also, as we have seen, an extremely general law
that the adult male differs more or less from the young male; and we may
conclude from this fact that the successive variations by which the adult male
became modified did not generally occur much before the age for reproduction.
Whenever some or many of the variations occurred early in life the young males
would partake more or less of the characters of the adult males; and
differences of this kind between the old and young males may be observed in
many species of animals.

     It is probable that young male animals have often tended to vary in a
manner which would not only have been of no use to them at an early age, but
would have been actually injurious - as by acquiring bright colors which would
render them conspicuous to their enemies, or by acquiring structures, such as
great horns, which would expend much vital force in their development.
Variations of this kind occurring in the young males would almost certainly be
eliminated through natural selection.  With the adult and experienced males,
on the other hand, the advantages derived from the acquisition of such
characters would more than counterbalance some exposure to danger and some
loss of vital force.

     As variations which give to the male a better chance of conquering other
males or of finding, securing or charming the opposite sex would, if they
happened to arise in the female, be of no service to her, they would not be
preserved in her through sexual selection.  We have also good evidence with
domesticated animals that variations of all kinds are, if not carefully
selected, soon lost through intercrossing and accidental deaths.  Consequently
in a state of nature if variations of the above kind chanced to arise in the
female line, and to be transmitted exclusively in this line, they would be
extremely liable to be lost.  If, however, the females varied and transmitted
their newly acquired characters to their offspring of both sexes the
characters which were advantageous to the males would be preserved by them
through sexual selection, and the two sexes would in consequence be modified
in the same manner, although such characters were of no use to the females;
but I shall hereafter have to recur to these more intricate contingencies.
Lastly, the females may acquire and apparently have often acquired by
transference characters from the male sex.

     As variations occurring late in life and transmitted to one sex alone
have incessantly been taken advantage of and accumulated through sexual
selection in relation to the reproduction of the species; therefore it
appears, at first sight, an unaccountable fact that similar variations have
not frequently been accumulated through natural selection, in relation to the
ordinary habits of life.  If this had occurred, the two sexes would often have
been differently modified, for the sake, for instance, of capturing prey or of
escaping from danger. Differences of this kind between the two sexes do
occasionally occur, especially in the lower classes.  But this implies that
the two sexes follow different habits in their struggles for existence, which
is a rare circumstance with the higher animals.  The case, however, is widely
different with the reproductive functions, in which respect the sexes
necessarily differ.  For variations in structure, which are related to these
functions, have often proved of value to one sex, and from having arisen at a
late period of life, have been transmitted to one sex alone; and such
variations, thus preserved and transmitted, have given rise to secondary
sexual characters.

     In the following chapters I shall treat of the secondary sexual
characters in animals of all classes, and shall endeavor in each case to apply
the principles explained in the present chapter.  The lowest classes will
detain us for a very short time, but the higher animals, especially birds,
must be treated at considerable length.  It should be borne in mind that for
reasons already assigned I intend to give only a few illustrative instances of
the innumerable structures by the aid of which the male finds the female, or,
when found, holds her.  On the other hand, all structures and instincts by the
aid of which the male conquers other males, and by which he allures or excites
the female, will be fully discussed, as these are in many ways the most
interesting.

Supplement On The Proportional Numbers Of The Two Sexes In Animals Belonging
To Various Classes.

     As no one, as far as I can discover, has paid attention to the relative
numbers of the two sexes throughout the animal kingdom, I will here give such
materials as I have been able to collect, although they are extremely
imperfect.  They consist in only a few instances of actual enumeration and the
numbers are not very large.  As the proportions are known with certainty only
in mankind, I will first give them as a standard of comparison.

     Man.  - In England during ten years (from 1857 to 1866) the average
number of children born alive yearly was 707, 120, in the proportion of 104.5
males to 100 females.  But in 1857 the male births throughout England were as
105.2, and in 1865 as 104 to 100.  Looking to separate districts, in
Buckinghamshire (where about 5,000 children are annually born) the mean
proportion of male to female births during the whole period of the above ten
years was as 102.8 to 100; while in N. Wales (where the average annual births
are 12,873) it was as high as 106.2 to 100.  Taking a still smaller district,
viz., Rutlandshire (where the annual births average only 739), in 1864 the
male births were as 114.6, and in 1862 as only 97 to 100; but even in this
small district the average of the 7,385 births during the whole ten years was
as 104.5 to 100; that is in the same ratio as throughout England. ^480 The
proportions are sometimes slightly disturbed by unknown causes; thus Prof.
Faye states "that in some districts of Norway there has been during a
decennial period a steady deficiency of boys, while in others the opposite
condition has existed." In France, during forty-four years, the male to the
female births have been as 106.2 to 100; but during this period it has
occurred five times in one department and six times in another, that the
female births have exceeded the males.  In Russia the average proportion is as
high as 108.9, and in Philadelphia, in the United States, as 110.5 to 100.
^481 The average for Europe, deduced by Bickes from about 70,000,000 births,
is 106 males to 100 females.  On the other hand, with white children born at
the Cape of Good Hope, the proportion of males is so low as to fluctuate
during successive years between 90 and 99 males for every 100 females.  It is
a singular fact that with Jews the proportion of male births is decidedly
large than with Christians; thus in Prussia the proportion is as 113, in
Breslau as 114, and in Livonia as 120 to 100; the Christian births in these
countries being the same as usual, for instance, in Livonia as 104 to 100.
^482

[Footnote 480: "Twenty-ninth Annual Report of the Registrar-General for 1866."
In this report (p. 12) a special decennial table is given.]

[Footnote 481: For Norway and Russia, see abstract of Prof. Faye's researches
in "British and Foreign Medico-Chirurg. Review," April, 1867, pp. 343, 345.
For France, the "Annuaire pour l'An, 1867," p. 213.  For Philadelphia, Dr.
Stockton Hough, "Social Science Assoc.," 1874.  For the Cape of Good Hope,
Quetelet as quoted by Dr. H. H. Zouteveen in the Dutch translation of this
work (vol. i, p. 417), where much information is given on the proportion of
the sexes.]

[Footnote 482: In regard to the Jews, see M. Thury, "La Loi de Production des
Sexes," 1863, p. 25.]

     Prof. Faye remarks that "a still greater preponderance of males would be
met with, if death struck both sexes in equal proportion in the womb and
during birth.  But the fact is, that for every 100 still-born females we have
in several countries from 134.6 to 144.9 still-born males.  During the first
four or five years of life, also, more male children die than females, for
example in England, during the first year, 126 boys die for every 100 girls -
a proportion which in France is still more unfavorable." ^483 Dr. Stockton
Hough accounts for these facts in part by the more frequent defective
development of males than of females. We have before seen that the male sex is
more variable in structure than the female; and variations in important organs
would generally be injurious.  But the size of the body, and especially of the
head, being greater in male than female infants is another cause; for the
males are thus more liable to be injured during parturition.  Consequently the
still-born males are more numerous; and as a highly competent judge, Dr.
Crichton Browne, ^484 believes male infants often suffer in health for some
years after birth.  Owing to this excess in the death-rate of male children,
both at birth and for some time subsequently, and owing to the exposure of
grown men to various dangers and to their tendency to emigrate, the females in
all old-settled countries, where statistical records have been kept, ^485 are
found to preponderate considerably over the males.

[Footnote 483: "British and Foreign Medico-Chirurg.  Review," April, 1867, p.
343.  Dr. Stark also remarks ("Tenth Annual Report of Births, Deaths, etc., in
Scotland," 1867, p. 28) that "These examples may suffice to show that, at
almost every stage of life, the males in Scotland have a greater liability to
death and a higher death-rate than the females.  The fact, however, of this
peculiarity being most strongly developed at that infantile period of life
when the dress, food and general treatment of both sexes are alike, seems to
prove that the higher male death-rate is an impressed, natural and
constitutional peculiarity due to sex alone."]

[Footnote 484: "West Riding Lunatic Asylum Reports," vol. i, 1871, p. 8. Sir
J. Simpson has proved that the head of the male infant exceeds that of the
female by three-eighths of an inch in circumference and by one-eighth in
transverse diameter.  Quetelet has shown that woman is born smaller than man;
see Dr. Duncan, "Fecundity, Fertility, Sterility," 1871, p. 382.]

[Footnote 485: With the savage Guaranys of Paraguay, according to the accurate
Azara ("Voyages dans l'Amerique merid., tom. ii, 1809, pp. 60, 179) the women
are to the men in the proportion of 14 to 13.]

     It seems at first sight a mysterious fact that in different nations,
under different conditions and climates, in Naples, Prussia, Westphalia,
Holland, France, England and the United States, the excess of male over female
births is less when they are illegitimate than when legitimate. ^486 This has
been explained by different writers in many different ways, as from the
mothers being generally young, from the large proportion of first pregnancies,
etc.  But we have seen that male infants, from the large size of their heads,
suffer more than female infants during parturition; and as the mothers of
illegitimate children must be more liable than other women to undergo bad
labors, from various causes, such as attempts at concealment by tight lacing,
hard work, distress of mind, etc., their male infants would proportionately
suffer. And this probably is the most efficient of all the causes of the
proportion of males to females born alive being less among illegitimate
children than among the legitimate.  With most animals the greater size of the
adult male than of the female is due to the stronger males having conquered
the weaker in their struggles for the possession of the females, and no doubt
it is owing to this fact that the two sexes of at least some animals differ in
size at birth.  Thus we have the curious fact that we may attribute the more
frequent deaths of male than female infants, especially among the
illegitimate, at least in part to sexual selection.

[Footnote 486: Babbage, "Edinburgh Journal of Science," 1829, vol. i, p. 88;
also p. 90, on still-born children.  On illegitimate children in England, see
"Report of Registrar-General for 1866," p. 15.]

     It has often been supposed that the relative age of the two parents
determines the sex of the offspring; and Prof. Leuckart ^487 has advanced what
he considers sufficient evidence, with respect to man and certain domesticated
animals, that this is one important though not the sole factor in the result.
So again the period of impregnation relatively to the state of the female has
been thought by some to be the efficient cause; but recent observations
discountenance this belief.  According to Dr. Stockton Hough, ^488 the season
of the year, the poverty or wealth of the parents, residence in the country or
in cities, the crossing of foreign immigrants, etc., all influence the
proportion of the sexes. With mankind, polygamy has also been supposed to lead
to the birth of a greater proportion of female infants; but Dr. J. Campbell
^489 carefully attended to this subject in the harems of Siam, and concludes
that the proportion of male to female births is the same as from monogamous
unions.  Hardly any animal has been rendered so highly polygamous as the
English race-horse, and we shall immediately see that his male and female
offspring are almost exactly equal in number.  I will now give the facts which
I have collected with respect to the proportional numbers of the sexes of
various animals; and will then briefly discuss how far selection has come into
play in determining the result.

[Footnote 487: Leuckart, in Wagner, "Handworterbuch der Phys.," B. iv, 1853,
s. 774.]

[Footnote 488: Social Science Assoc. of Philadelphia, 1874.]

[Footnote 489: "Anthropological Review," April, 1870, p. 108.]