whilst 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; whilst 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 (40. Antilocapra Americana. I have to thank Dr.
Canfield for information with respect to the horns of the female: see
also his paper in ‘Proceedings of the Zoological Society,’ 1866, p.
109. Also Owen, ‘Anatomy of Vertebrates,’ vol. iii. p. 627), 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 afterwards. (41. 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.) 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 enquiry (42. I am
greatly indebted to Prof. Victor Carus for having made enquiries 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 shewed 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.), 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.
Dr. W. Marshall has lately made a special study of the protuberances so
common on the heads of birds (43. ‘Über die knochernen Schädelhöcker
der Vögel,’ in the ‘Niederland. Archiv fur Zoologie,’ B.i. Heft 2,
1872.), 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.
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. (44. In the common peacock (Pavo cristatus) the male
alone possesses spurs, whilst 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 23rd, 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.) 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, whilst the curled tail-feathers and other ornaments of
the male are developed later. (45. 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 male 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
greyish-white in the female. Now the young males at first entirely
resemble the females, and have a greyish-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.) 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.
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 coloured scales, for instance, in two species of
butterflies, in one of which the sexes differ in colour, whilst 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 coloured marks are confined to one sex, whilst 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
colour-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 colour, 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 the 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 (46. ‘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.), in which both sexes change their colour
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.
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 colour 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 coloured in nearly the
same manner as their parents, and this renders it probable that their
colours 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 coloured. With the so-called
Cuckoo sub-breeds of the fowl, in which the feathers are transversely
pencilled with dark stripes, both sexes and the chickens are coloured
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 pencilled. With respect to other
characters besides colour, 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 (47. 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.); and in this breed the adults of both sexes are
characterised by a great bony protuberance and an immense crest.
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 efficient cause of characters being inherited by both sexes, is
their development at an early age, whilst 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.
A 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 ardour in 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 towards 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
colours, 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 later 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 endeavour 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 104.0 to 100. Looking to separate
districts, in Buckinghamshire (where about 5000 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; whilst 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 97.0 to 100; but even in this small district the
average of the 7385 births during the whole ten years, was as 104.5 to
100: that is in the same ratio as throughout England. (48.
‘Twenty-ninth Annual Report of the Registrar-General for 1866.’ In this
report (p. xii.) a special decennial table is given.) 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, whilst 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. (49. 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.) The
average for Europe, deduced by Bickes from about seventy million
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 larger 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. (50. 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 unfavourable.” (51. ‘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. xxviii.) 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.”) 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 (52. ‘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 3/8ths of an inch in circumference, and by 1/8th in
transverse diameter. Quetelet has shewn that woman is born smaller than
man; see Dr. Duncan, ‘Fecundity, Fertility, and Sterility,’ 1871, p.
382.), 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, are found to preponderate considerably over the males. (53.
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. (54. 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. xv.) 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 labours, from various causes,
such as attempts at concealment by tight lacing, hard work, distress of
mind, etc., their male infants would proportionably suffer. And this
probably is the most efficient of all the causes of the proportion of
males to females born alive being less amongst illegitimate children
than amongst 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 amongst the illegitimate, at least in part to
sexual selection.
It has often been supposed that the relative age of the two parents
determine the sex of the offspring; and Prof. Leuckart (55. Leuckart,
in Wagner ‘Handwörterbuch der Phys.’ B. iv. 1853, s. 774.) 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 (56. ‘Social Science Association of Philadelphia,’
1874.), 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 (57.
‘Anthropological Review,’ April 1870, p. cviii.) 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.
HORSES.
Mr. Tegetmeier has been so kind as to tabulate for me from the ‘Racing
Calendar’ the births of race-horses during a period of twenty-one
years, viz., from 1846 to 1867; 1849 being omitted, as no returns were
that year published. The total births were 25,560 (58. During eleven
years a record was kept of the number of mares which proved barren or
prematurely slipped their foals; and it deserves notice, as shewing how
infertile these highly-nurtured and rather closely-interbred animals
have become, that not far from one-third of the mares failed to produce
living foals. Thus during 1866, 809 male colts and 816 female colts
were born, and 743 mares failed to produce offspring. During 1867, 836
males and 902 females were born, and 794 mares failed.), consisting of
12,763 males and 12,797 females, or in the proportion of 99.7 males to
100 females. As these numbers are tolerably large, and as they are
drawn from all parts of England, during several years, we may with much
confidence conclude that with the domestic horse, or at least with the
race-horse, the two sexes are produced in almost equal numbers. The
fluctuations in the proportions during successive years are closely
like those which occur with mankind, when a small and thinly-populated
area is considered; thus in 1856 the male horses were as 107.1, and in
1867 as only 92.6 to 100 females. In the tabulated returns the
proportions vary in cycles, for the males exceeded the females during
six successive years; and the females exceeded the males during two
periods each of four years; this, however, may be accidental; at least
I can detect nothing of the kind with man in the decennial table in the
Registrar’s Report for 1866.
DOGS.
During a period of twelve years, from 1857 to 1868, the births of a
large number of greyhounds, throughout England, were sent to the
‘Field’ newspaper; and I am again indebted to Mr. Tegetmeier for
carefully tabulating the results. The recorded births were 6878,
consisting of 3605 males and 3273 females, that is, in the proportion
of 110.1 males to 100 females. The greatest fluctuations occurred in
1864, when the proportion was as 95.3 males, and in 1867, as 116.3
males to 100 females. The above average proportion of 110.1 to 100 is
probably nearly correct in the case of the greyhound, but whether it
would hold with other domesticated breeds is in some degree doubtful.
Mr. Cupples has enquired from several great breeders of dogs, and finds
that all without exception believe that females are produced in excess;
but he suggests that this belief may have arisen from females being
less valued, and from the consequent disappointment producing a
stronger impression on the mind.
SHEEP.
The sexes of sheep are not ascertained by agriculturists until several
months after birth, at the period when the males are castrated; so that
the following returns do not give the proportions at birth. Moreover, I
find that several great breeders in Scotland, who annually raise some
thousand sheep, are firmly convinced that a larger proportion of males
than of females die during the first year or two. Therefore the
proportion of males would be somewhat larger at birth than at the age
of castration. This is a remarkable coincidence with what, as we have
seen, occurs with mankind, and both cases probably depend on the same
cause. I have received returns from four gentlemen in England who have
bred Lowland sheep, chiefly Leicesters, during the last ten to sixteen
years; they amount altogether to 8965 births, consisting of 4407 males
and 4558 females; that is in the proportion of 96.7 males to 100
females. With respect to Cheviot and black-faced sheep bred in
Scotland, I have received returns from six breeders, two of them on a
large scale, chiefly for the years 1867-1869, but some of the returns
extend back to 1862. The total number recorded amounts to 50,685,
consisting of 25,071 males and 25,614 females or in the proportion of
97.9 males to 100 females. If we take the English and Scotch returns
together, the total number amounts to 59,650, consisting of 29,478
males and 30,172 females, or as 97.7 to 100. So that with sheep at the
age of castration the females are certainly in excess of the males, but
probably this would not hold good at birth. (59. I am much indebted to
Mr. Cupples for having procured for me the above returns from Scotland,
as well as some of the following returns on cattle. Mr. R. Elliot, of
Laighwood, first called my attention to the premature deaths of the
males, —a statement subsequently confirmed by Mr. Aitchison and others.
To this latter gentleman, and to Mr. Payan, I owe my thanks for large
returns as to sheep.)
Of CATTLE I have received returns from nine gentlemen of 982 births,
too few to be trusted; these consisted of 477 bull-calves and 505
cow-calves; i.e., in the proportion of 94.4 males to 100 females. The
Rev. W.D. Fox informs me that in 1867 out of 34 calves born on a farm
in Derbyshire only one was a bull. Mr. Harrison Weir has enquired from
several breeders of PIGS, and most of them estimate the male to the
female births as about 7 to 6. This same gentleman has bred RABBITS for
many years, and has noticed that a far greater number of bucks are
produced than does. But estimations are of little value.
Of mammalia in a state of nature I have been able to learn very little.
In regard to the common rat, I have received conflicting statements.
Mr. R. Elliot, of Laighwood, informs me that a rat-catcher assured him
that he had always found the males in great excess, even with the young
in the nest. In consequence of this, Mr. Elliot himself subsequently
examined some hundred old ones, and found the statement true. Mr. F.
Buckland has bred a large number of white rats, and he also believes
that the males greatly exceed the females. In regard to Moles, it is
said that “the males are much more numerous than the females” (60.
Bell, ‘History of British Quadrupeds,’ p. 100.): and as the catching of
these animals is a special occupation, the statement may perhaps be
trusted. Sir A. Smith, in describing an antelope of S. Africa (61.
‘Illustrations of the Zoology of S. Africa,’ 1849, pl. 29.) (Kobus
ellipsiprymnus), remarks, that in the herds of this and other species,
the males are few in number compared with the females: the natives
believe that they are born in this proportion; others believe that the
younger males are expelled from the herds, and Sir A. Smith says, that
though he has himself never seen herds consisting of young males alone,
others affirm that this does occur. It appears probable that the young
when expelled from the herd, would often fall a prey to the many beasts
of prey of the country.
BIRDS.
With respect to the FOWL, I have received only one account, namely,
that out of 1001 chickens of a highly-bred stock of Cochins, reared
during eight years by Mr. Stretch, 487 proved males and 514 females;
i.e., as 94.7 to 100. In regard to domestic pigeons there is good
evidence either that the males are produced in excess, or that they
live longer; for these birds invariably pair, and single males, as Mr.
Tegetmeier informs me, can always be purchased cheaper than females.
Usually the two birds reared from the two eggs laid in the same nest
are a male and a female; but Mr. Harrison Weir, who has been so large a
breeder, says that he has often bred two cocks from the same nest, and
seldom two hens; moreover, the hen is generally the weaker of the two,
and more liable to perish.
With respect to birds in a state of nature, Mr. Gould and others (62.
Brehm (‘Thierleben,’ B. iv. s. 990) comes to the same conclusion.) are
convinced that the males are generally the more numerous; and as the
young males of many species resemble the females, the latter would
naturally appear to be the more numerous. Large numbers of pheasants
are reared by Mr. Baker of Leadenhall from eggs laid by wild birds, and
he informs Mr. Jenner Weir that four or five males to one female are
generally produced. An experienced observer remarks (63. On the
authority of L. Lloyd, ‘Game Birds of Sweden,’ 1867, pp. 12, 132.),
that in Scandinavia the broods of the capercailzie and black-cock
contain more males than females; and that with the Dal-ripa (a kind of
ptarmigan) more males than females attend the leks or places of
courtship; but this latter circumstance is accounted for by some
observers by a greater number of hen birds being killed by vermin. From
various facts given by White of Selborne (64. ‘Nat. Hist. of Selborne,’
letter xxix. edit. of 1825, vol. i. p. 139.), it seems clear that the
males of the partridge must be in considerable excess in the south of
England; and I have been assured that this is the case in Scotland. Mr.
Weir on enquiring from the dealers, who receive at certain seasons
large numbers of ruffs (Machetes pugnax), was told that the males are
much the more numerous. This same naturalist has also enquired for me
from the birdcatchers, who annually catch an astonishing number of
various small species alive for the London market, and he was
unhesitatingly answered by an old and trustworthy man, that with the
chaffinch the males are in large excess: he thought as high as 2 males
to 1 female, or at least as high as 5 to 3. (65. Mr. Jenner Weir
received similar information, on making enquiries during the following
year. To shew the number of living chaffinches caught, I may mention
that in 1869 there was a match between two experts, and one man caught
in a day 62, and another 40, male chaffinches. The greatest number ever
caught by one man in a single day was 70.) The males of the blackbird,
he likewise maintained, were by far the more numerous, whether caught
by traps or by netting at night. These statements may apparently be
trusted, because this same man said that the sexes are about equal with
the lark, the twite (Linaria montana), and goldfinch. On the other
hand, he is certain that with the common linnet, the females
preponderate greatly, but unequally during different years; during some
years he has found the females to the males as four to one. It should,
however, be borne in mind, that the chief season for catching birds
does not begin till September, so that with some species partial
migrations may have begun, and the flocks at this period often consist
of hens alone. Mr. Salvin paid particular attention to the sexes of the
humming-birds in Central America, and is convinced that with most of
the species the males are in excess; thus one year he procured 204
specimens belonging to ten species, and these consisted of 166 males
and of only 38 females. With two other species the females were in
excess: but the proportions apparently vary either during different
seasons or in different localities; for on one occasion the males of
Campylopterus hemileucurus were to the females as 5 to 2, and on
another occasion (66. ‘Ibis,’ vol. ii. p. 260, as quoted in Gould’s
‘Trochilidae,’ 1861, p. 52. For the foregoing proportions, I am
indebted to Mr. Salvin for a table of his results.) in exactly the
reversed ratio. As bearing on this latter point, I may add, that Mr.
Powys found in Corfu and Epirus the sexes of the chaffinch keeping
apart, and “the females by far the most numerous”; whilst in Palestine
Mr. Tristram found “the male flocks appearing greatly to exceed the
female in number.” (67. ‘Ibis,’ 1860, p. 137; and 1867, p. 369.) So
again with the Quiscalus major, Mr. G. Taylor says, that in Florida
there were “very few females in proportion to the males,” (68. ‘Ibis,’
1862, p. 187.) whilst in Honduras the proportion was the other way, the
species there having the character of a polygamist.
FISH.
With fish the proportional numbers of the sexes can be ascertained only
by catching them in the adult or nearly adult state; and there are many
difficulties in arriving at any just conclusion. (69. Leuckart quotes
Bloch (Wagner, ‘Handwörterbuch der Phys.’ B. iv. 1853, s. 775), that
with fish there are twice as many males as females.) Infertile females
might readily be mistaken for males, as Dr. Gunther has remarked to me
in regard to trout. With some species the males are believed to die
soon after fertilising the ova. With many species the males are of much
smaller size than the females, so that a large number of males would
escape from the same net by which the females were caught. M.
Carbonnier (70. Quoted in the ‘Farmer,’ March 18, 1869, p. 369.), who
has especially attended to the natural history of the pike (Esox
lucius), states that many males, owing to their small size, are
devoured by the larger females; and he believes that the males of
almost all fish are exposed from this same cause to greater danger than
the females. Nevertheless, in the few cases in which the proportional
numbers have been actually observed, the males appear to be largely in
excess. Thus Mr. R. Buist, the superintendent of the Stormontfield
experiments, says that in 1865, out of 70 salmon first landed for the
purpose of obtaining the ova, upwards of 60 were males. In 1867 he
again “calls attention to the vast disproportion of the males to the
females. We had at the outset at least ten males to one female.”
Afterwards females sufficient for obtaining ova were procured. He adds,
“from the great proportion of the males, they are constantly fighting
and tearing each other on the spawning-beds.” (71. ‘The Stormontfield
Piscicultural Experiments,’ 1866, p. 23. The ‘Field’ newspaper, June
29, 1867.) This disproportion, no doubt, can be accounted for in part,
but whether wholly is doubtful, by the males ascending the rivers
before the females. Mr. F. Buckland remarks in regard to trout, that
“it is a curious fact that the males preponderate very largely in
number over the females. It INVARIABLY happens that when the first rush
of fish is made to the net, there will be at least seven or eight males
to one female found captive. I cannot quite account for this; either
the males are more numerous than the females, or the latter seek safety
by concealment rather than flight.” He then adds, that by carefully
searching the banks sufficient females for obtaining ova can be found.
(72. ‘Land and Water,’ 1868, p. 41.) Mr. H. Lee informs me that out of
212 trout, taken for this purpose in Lord Portsmouth’s park, 150 were
males and 62 females.
The males of the Cyprinidae likewise seem to be in excess; but several
members of this Family, viz., the carp, tench, bream and minnow, appear
regularly to follow the practice, rare in the animal kingdom, of
polyandry; for the female whilst spawning is always attended by two
males, one on each side, and in the case of the bream by three or four
males. This fact is so well known, that it is always recommended to
stock a pond with two male tenches to one female, or at least with
three males to two females. With the minnow, an excellent observer
states, that on the spawning-beds the males are ten times as numerous
as the females; when a female comes amongst the males, “she is
immediately pressed closely by a male on each side; and when they have
been in that situation for a time, are superseded by other two males.”
(73. Yarrell, ‘Hist. British Fishes,’ vol. i. 1826, p. 307; on the
Cyprinus carpio, p. 331; on the Tinca vulgaris, p. 331; on the Abramis
brama, p. 336. See, for the minnow (Leuciscus phoxinus), ‘Loudon’s
Magazine of Natural History,’ vol. v. 1832, p. 682.)
INSECTS.
In this great Class, the Lepidoptera almost alone afford means for
judging of the proportional numbers of the sexes; for they have been
collected with special care by many good observers, and have been
largely bred from the egg or caterpillar state. I had hoped that some
breeders of silk-moths might have kept an exact record, but after
writing to France and Italy, and consulting various treatises, I cannot
find that this has ever been done. The general opinion appears to be
that the sexes are nearly equal, but in Italy, as I hear from Professor
Canestrini, many breeders are convinced that the females are produced
in excess. This same naturalist, however, informs me, that in the two
yearly broods of the Ailanthus silk-moth (Bombyx cynthia), the males
greatly preponderate in the first, whilst in the second the two sexes
are nearly equal, or the females rather in excess.
In regard to Butterflies in a state of nature, several observers have
been much struck by the apparently enormous preponderance of the males.
(74. Leuckart quotes Meinecke (Wagner, ‘Handwörterbuch der Phys.’ B.
iv. 1853, s. 775) that the males of Butterflies are three or four times
as numerous as the females.) Thus Mr. Bates (75. ‘The Naturalist on the
Amazons,’ vol. ii. 1863, pp. 228, 347.), in speaking of several
species, about a hundred in number, which inhabit the upper Amazons,
says that the males are much more numerous than the females, even in
the proportion of a hundred to one. In North America, Edwards, who had
great experience, estimates in the genus Papilio the males to the
females as four to one; and Mr. Walsh, who informed me of this
statement, says that with P. turnus this is certainly the case. In
South Africa, Mr. R. Trimen found the males in excess in 19 species
(76. Four of these cases are given by Mr. Trimen in his ‘Rhopalocera
Africae Australis.’); and in one of these, which swarms in open places,
he estimated the number of males as fifty to one female. With another
species, in which the males are numerous in certain localities, he
collected only five females during seven years. In the island of
Bourbon, M. Maillard states that the males of one species of Papilio
are twenty times as numerous as the females. (77. Quoted by Trimen,
‘Transactions of the Ent. Society,’ vol. v. part iv. 1866, p. 330.) Mr.
Trimen informs me that as far as he has himself seen, or heard from
others, it is rare for the females of any butterfly to exceed the males
in number; but three South African species perhaps offer an exception.
Mr. Wallace (78. ‘Transactions, Linnean Society,’ vol. xxv. p. 37.)
states that the females of Ornithoptera croesus, in the Malay
archipelago, are more common and more easily caught than the males; but
this is a rare butterfly. I may here add, that in Hyperythra, a genus
of moths, Guenee says, that from four to five females are sent in
collections from India for one male.
When this subject of the proportional numbers of the sexes of insects
was brought before the Entomological Society (79. ‘Proceedings,
Entomological Society,’ Feb. 17, 1868.), it was generally admitted that
the males of most Lepidoptera, in the adult or imago state, are caught
in greater numbers than the females: but this fact was attributed by
various observers to the more retiring habits of the females, and to
the males emerging earlier from the cocoon. This latter circumstance is
well known to occur with most Lepidoptera, as well as with other
insects. So that, as M. Personnat remarks, the males of the
domesticated Bombyx Yamamai, are useless at the beginning of the
season, and the females at the end, from the want of mates. (80. Quoted
by Dr. Wallace in ‘Proceedings, Entomological Society,’ 3rd series,
vol. v. 1867, p. 487.) I cannot, however, persuade myself that these
causes suffice to explain the great excess of males, in the above cases
of certain butterflies which are extremely common in their native
countries. Mr. Stainton, who has paid very close attention during many
years to the smaller moths, informs me that when he collected them in
the imago state, he thought that the males were ten times as numerous
as the females, but that since he has reared them on a large scale from
the caterpillar state, he is convinced that the females are the more
numerous. Several entomologists concur in this view. Mr. Doubleday,
however, and some others, take an opposite view, and are convinced that
they have reared from the eggs and caterpillars a larger proportion of
males than of females.
Besides the more active habits of the males, their earlier emergence
from the cocoon, and in some cases their frequenting more open
stations, other causes may be assigned for an apparent or real
difference in the proportional numbers of the sexes of Lepidoptera,
when captured in the imago state, and when reared from the egg or
caterpillar state. I hear from Professor Canestrini, that it is
believed by many breeders in Italy, that the female caterpillar of the
silk-moth suffers more from the recent disease than the male; and Dr.
Staudinger informs me that in rearing Lepidoptera more females die in
the cocoon than males. With many species the female caterpillar is
larger than the male, and a collector would naturally choose the finest
specimens, and thus unintentionally collect a larger number of females.
Three collectors have told me that this was their practice; but Dr.
Wallace is sure that most collectors take all the specimens which they
can find of the rarer kinds, which alone are worth the trouble of
rearing. Birds when surrounded by caterpillars would probably devour
the largest; and Professor Canestrini informs me that in Italy some
breeders believe, though on insufficient evidence, that in the first
broods of the Ailanthus silk-moth, the wasps destroy a larger number of
the female than of the male caterpillars. Dr. Wallace further remarks
that female caterpillars, from being larger than the males, require
more time for their development, and consume more food and moisture:
and thus they would be exposed during a longer time to danger from
ichneumons, birds, etc., and in times of scarcity would perish in
greater numbers. Hence it appears quite possible that in a state of
nature, fewer female Lepidoptera may reach maturity than males; and for
our special object we are concerned with their relative numbers at
maturity, when the sexes are ready to propagate their kind.
The manner in which the males of certain moths congregate in
extraordinary numbers round a single female, apparently indicates a
great excess of males, though this fact may perhaps be accounted for by
the earlier emergence of the males from their cocoons. Mr. Stainton
informs me that from twelve to twenty males, may often be seen
congregated round a female Elachista rufocinerea. It is well known that
if a virgin Lasiocampa quercus or Saturnia carpini be exposed in a
cage, vast numbers of males collect round her, and if confined in a
room will even come down the chimney to her. Mr. Doubleday believes
that he has seen from fifty to a hundred males of both these species
attracted in the course of a single day by a female in confinement. In
the Isle of Wight Mr. Trimen exposed a box in which a female of the
Lasiocampa had been confined on the previous day, and five males soon
endeavoured to gain admittance. In Australia, Mr. Verreaux, having
placed the female of a small Bombyx in a box in his pocket, was
followed by a crowd of males, so that about 200 entered the house with
him. (81. Blanchard, ‘Metamorphoses, Moeurs des Insectes,’ 1868, pp.
225-226.)
Mr. Doubleday has called my attention to M. Staudinger’s (82.
‘Lepidopteren-Doubletten Liste,’ Berlin, No. x. 1866.) list of
Lepidoptera, which gives the prices of the males and females of 300
species or well-marked varieties of butterflies (Rhopalocera). The
prices for both sexes of the very common species are of course the
same; but in 114 of the rarer species they differ; the males being in
all cases, excepting one, the cheaper. On an average of the prices of
the 113 species, the price of the male to that of the female is as 100
to 149; and this apparently indicates that inversely the males exceed
the females in the same proportion. About 2000 species or varieties of
moths (Heterocera) are catalogued, those with wingless females being
here excluded on account of the difference in habits between the two
sexes: of these 2000 species, 141 differ in price according to sex, the
males of 130 being cheaper, and those of only 11 being dearer than the
females. The average price of the males of the 130 species, to that of
the females, is as 100 to 143. With respect to the butterflies in this
priced list, Mr. Doubleday thinks (and no man in England has had more
experience), that there is nothing in the habits of the species which
can account for the difference in the prices of the two sexes, and that
it can be accounted for only by an excess in the number of the males.
But I am bound to add that Dr. Staudinger informs me, that he is
himself of a different opinion. He thinks that the less active habits
of the females and the earlier emergence of the males will account for
his collectors securing a larger number of males than of females, and
consequently for the lower prices of the former. With respect to
specimens reared from the caterpillar-state, Dr. Staudinger believes,
as previously stated, that a greater number of females than of males
die whilst confined to the cocoons. He adds that with certain species
one sex seems to preponderate over the other during certain years.
Of direct observations on the sexes of Lepidoptera, reared either from
eggs or caterpillars, I have received only the few following cases:
(See following table.)
So that in these eight lots of cocoons and eggs, males were produced in
excess. Taken together the proportion of males is as 122.7 to 100
females. But the numbers are hardly large enough to be trustworthy.
On the whole, from these various sources of evidence, all pointing in
the same direction, I infer that with most species of Lepidoptera, the
mature males generally exceed the females in number, whatever the
proportions may be at their first emergence from the egg.
Males Females
The Rev. J. Hellins* of Exeter reared, during
1868, imagos of 73 species, which
consisted of 153 137
Mr. Albert Jones of Eltham reared, during
1868, imagos of 9 species, which
consisted of 159 126
During 1869 he reared imagos from 4 species
consisting of 114 112
Mr. Buckler of Emsworth, Hants, during 1869,
reared imagos from 74 species,
consisting of 180 169
Dr. Wallace of Colchester reared from one
brood of Bombyx cynthia 52 48
Dr. Wallace raised, from cocoons of Bombyx
Pernyi sent from China, during 1869 224 123
Dr. Wallace raised, during 1868 and 1869, from
two lots of cocoons of Bombyx yamamai 52 46
Total 934 761
(*83. This naturalist has been so kind as to send me some results from
former years, in which the females seemed to preponderate; but so many
of the figures were estimates, that I found it impossible to tabulate
them.)
With reference to the other Orders of insects, I have been able to
collect very little reliable information. With the stag-beetle (Lucanus
cervus) “the males appear to be much more numerous than the females”;
but when, as Cornelius remarked during 1867, an unusual number of these
beetles appeared in one part of Germany, the females appeared to exceed
the males as six to one. With one of the Elateridae, the males are said
to be much more numerous than the females, and “two or three are often
found united with one female (84. Gunther’s ‘Record of Zoological
Literature,’ 1867, p. 260. On the excess of female Lucanus, ibid, p.
250. On the males of Lucanus in England, Westwood,’ ‘Modern
Classification of Insects,’ vol. i. p. 187. On the Siagonium, ibid. p.
172.); so that here polyandry seems to prevail.” With Siagonium
(Staphylinidae), in which the males are furnished with horns, “the
females are far more numerous than the opposite sex.” Mr. Janson stated
at the Entomological Society that the females of the bark feeding
Tomicus villosus are so common as to be a plague, whilst the males are
so rare as to be hardly known.
It is hardly worth while saying anything about the proportion of the
sexes in certain species and even groups of insects, for the males are
unknown or very rare, and the females are parthenogenetic, that is,
fertile without sexual union; examples of this are afforded by several
of the Cynipidae. (85. Walsh in ‘The American Entomologist,’ vol. i.
1869, p. 103. F. Smith, ‘Record of Zoological Lit.’ 1867, p. 328.) In
all the gall-making Cynipidae known to Mr. Walsh, the females are four
or five times as numerous as the males; and so it is, as he informs me,
with the gall-making Cecidomyiidae (Diptera). With some common species
of Saw-flies (Tenthredinae) Mr. F. Smith has reared hundreds of
specimens from larvae of all sizes, but has never reared a single male;
on the other hand, Curtis says (86. ‘Farm Insects,’ pp. 45-46.), that
with certain species (Athalia), bred by him, the males were to the
females as six to one; whilst exactly the reverse occurred with the
mature insects of the same species caught in the fields. In the family
of bees, Hermann Müller (87. ‘Anwendung der Darwin’schen Lehre,’ Verh.
d. n. Jahrg., xxiv.), collected a large number of specimens of many
species, and reared others from the cocoons, and counted the sexes. He
found that the males of some species greatly exceeded the females in
number; in others the reverse occurred; and in others the two sexes
were nearly equal. But as in most cases the males emerge from the
cocoons before the females, they are at the commencement of the
breeding-season practically in excess. Müller also observed that the
relative number of the two sexes in some species differed much in
different localities. But as H. Müller has himself remarked to me,
these remarks must be received with some caution, as one sex might more
easily escape observation than the other. Thus his brother Fritz Müller
has noticed in Brazil that the two sexes of the same species of bee
sometimes frequent different kinds of flowers. With respect to the
Orthoptera, I know hardly anything about the relative number of the
sexes: Korte (88. ‘Die Strich, Zug oder Wanderheuschrecke,’ 1828, p.
20.), however, says that out of 500 locusts which he examined, the
males were to the females as five to six. With the Neuroptera, Mr.
Walsh states that in many, but by no means in all the species of the
Odonatous group, there is a great overplus of males: in the genus
Hetaerina, also, the males are generally at least four times as
numerous as the females. In certain species in the genus Gomphus the
males are equally in excess, whilst in two other species, the females
are twice or thrice as numerous as the males. In some European species
of Psocus thousands of females may be collected without a single male,
whilst with other species of the same genus both sexes are common. (89.
‘Observations on N. American Neuroptera,’ by H. Hagen and B.D. Walsh,
‘Proceedings, Ent. Soc. Philadelphia,’ Oct. 1863, pp. 168, 223, 239.)
In England, Mr. MacLachlan has captured hundreds of the female Apatania
muliebris, but has never seen the male; and of Boreus hyemalis only
four or five males have been seen here. (90. ‘Proceedings, Ent. Soc.
London,’ Feb. 17, 1868.) With most of these species (excepting the
Tenthredinae) there is at present no evidence that the females are
subject to parthenogenesis; and thus we see how ignorant we are of the
causes of the apparent discrepancy in the proportion of the two sexes.
In the other classes of the Articulata I have been able to collect
still less information. With spiders, Mr. Blackwall, who has carefully
attended to this class during many years, writes to me that the males
from their more erratic habits are more commonly seen, and therefore
appear more numerous. This is actually the case with a few species; but
he mentions several species in six genera, in which the females appear
to be much more numerous than the males. (91. Another great authority
with respect to this class, Prof. Thorell of Upsala (‘On European
Spiders,’ 1869-70, part i. p. 205), speaks as if female spiders were
generally commoner than the males.) The small size of the males in
comparison with the females (a peculiarity which is sometimes carried
to an extreme degree), and their widely different appearance, may
account in some instances for their rarity in collections. (92. See, on
this subject, Mr. O.P. Cambridge, as quoted in ‘Quarterly Journal of
Science,’ 1868, page 429.)
Some of the lower Crustaceans are able to propagate their kind
sexually, and this will account for the extreme rarity of the males;
thus von Siebold (93. ‘Beiträge zur Parthenogenesis,’ p. 174.)
carefully examined no less than 13,000 specimens of Apus from
twenty-one localities, and amongst these he found only 319 males. With
some other forms (as Tanais and Cypris), as Fritz Müller informs me,
there is reason to believe that the males are much shorter-lived than
the females; and this would explain their scarcity, supposing the two
sexes to be at first equal in number. On the other hand, Müller has
invariably taken far more males than females of the Diastylidae and of
Cypridina on the shores of Brazil: thus with a species in the latter
genus, 63 specimens caught the same day included 57 males; but he
suggests that this preponderance may be due to some unknown difference
in the habits of the two sexes. With one of the higher Brazilian crabs,
namely a Gelasimus, Fritz Müller found the males to be more numerous
than the females. According to the large experience of Mr. C. Spence
Bate, the reverse seems to be the case with six common British crabs,
the names of which he has given me.
THE PROPORTION OF THE SEXES IN RELATION TO NATURAL SELECTION.
There is reason to suspect that in some cases man has by selection
indirectly influenced his own sex-producing powers. Certain women tend
to produce during their whole lives more children of one sex than of
the other: and the same holds good of many animals, for instance, cows
and horses; thus Mr. Wright of Yeldersley House informs me that one of
his Arab mares, though put seven times to different horses, produced
seven fillies. Though I have very little evidence on this head, analogy
would lead to the belief, that the tendency to produce either sex would
be inherited like almost every other peculiarity, for instance, that of
producing twins; and concerning the above tendency a good authority,
Mr. J. Downing, has communicated to me facts which seem to prove that
this does occur in certain families of short-horn cattle. Col. Marshall
(94. ‘The Todas,’ 1873, pp. 100, 111, 194, 196.) has recently found on
careful examination that the Todas, a hill-tribe of India, consist of
112 males and 84 females of all ages—that is in a ratio of 133.3 males
to 100 females. The Todas, who are polyandrous in their marriages,
during former times invariably practised female infanticide; but this
practice has now been discontinued for a considerable period. Of the
children born within late years, the males are more numerous than the
females, in the proportion of 124 to 100. Colonel Marshall accounts for
this fact in the following ingenious manner. “Let us for the purpose of
illustration take three families as representing an average of the
entire tribe; say that one mother gives birth to six daughters and no
sons; a second mother has six sons only, whilst the third mother has
three sons and three daughters. The first mother, following the tribal
custom, destroys four daughters and preserves two. The second retains
her six sons. The third kills two daughters and keeps one, as also her
three sons. We have then from the three families, nine sons and three
daughters, with which to continue the breed. But whilst the males
belong to families in which the tendency to produce sons is great, the
females are of those of a converse inclination. Thus the bias
strengthens with each generation, until, as we find, families grow to
have habitually more sons than daughters.”
That this result would follow from the above form of infanticide seems
almost certain; that is if we assume that a sex-producing tendency is
inherited. But as the above numbers are so extremely scanty, I have
searched for additional evidence, but cannot decide whether what I have
found is trustworthy; nevertheless the facts are, perhaps, worth
giving. The Maories of New Zealand have long practised infanticide; and
Mr. Fenton (95. ‘Aboriginal Inhabitants of New Zealand: Government
Report,’ 1859, p. 36.) states that he “has met with instances of women
who have destroyed four, six, and even seven children, mostly females.
However, the universal testimony of those best qualified to judge, is
conclusive that this custom has for many years been almost extinct.
Probably the year 1835 may be named as the period of its ceasing to
exist.” Now amongst the New Zealanders, as with the Todas, male births
are considerably in excess. Mr. Fenton remarks (p. 30), “One fact is
certain, although the exact period of the commencement of this singular
condition of the disproportion of the sexes cannot be demonstratively
fixed, it is quite clear that this course of decrease was in full
operation during the years 1830 to 1844, when the non-adult population
of 1844 was being produced, and has continued with great energy up to
the present time.” The following statements are taken from Mr. Fenton
(p. 26), but as the numbers are not large, and as the census was not
accurate, uniform results cannot be expected. It should be borne in
mind in this and the following cases, that the normal state of every
population is an excess of women, at least in all civilised countries,
chiefly owing to the greater mortality of the male sex during youth,
and partly to accidents of all kinds later in life. In 1858, the native
population of New Zealand was estimated as consisting of 31,667 males
and 24,303 females of all ages, that is in the ratio of 130.3 males to
100 females. But during this same year, and in certain limited
districts, the numbers were ascertained with much care, and the males
of all ages were here 753 and the females 616; that is in the ratio of
122.2 males to 100 females. It is more important for us that during
this same year of 1858, the NON-ADULT males within the same district
were found to be 178, and the NON-ADULT females 142, that is in the
ratio of 125.3 to 100. It may be added that in 1844, at which period
female infanticide had only lately ceased, the NON-ADULT males in one
district were 281, and the NON-ADULT females only 194, that is in the
ratio of 144.8 males to 100 females.
In the Sandwich Islands, the males exceed the females in number.
Infanticide was formerly practised there to a frightful extent, but was
by no means confined to female infants, as is shewn by Mr. Ellis (96.
‘Narrative of a Tour through Hawaii,’ 1826, p. 298.), and as I have
been informed by Bishop Staley and the Rev. Mr. Coan. Nevertheless,
another apparently trustworthy writer, Mr. Jarves (97. ‘History of the
Sandwich Islands,’ 1843, p. 93.), whose observations apply to the whole
archipelago, remarks:—“Numbers of women are to be found, who confess to
the murder of from three to six or eight children,” and he adds,
“females from being considered less useful than males were more often
destroyed.” From what is known to occur in other parts of the world,
this statement is probable; but must be received with much caution. The
practice of infanticide ceased about the year 1819, when idolatry was
abolished and missionaries settled in the Islands. A careful census in
1839 of the adult and taxable men and women in the island of Kauai and
in one district of Oahu (Jarves, p. 404), gives 4723 males and 3776
females; that is in the ratio of 125.08 to 100. At the same time the
number of males under fourteen years in Kauai and under eighteen in
Oahu was 1797, and of females of the same ages 1429; and here we have
the ratio of 125.75 males to 100 females.
In a census of all the islands in 1850 (98. This is given in the Rev.
H.T. Cheever’s ‘Life in the Sandwich Islands,’ 1851, p. 277.), the
males of all ages amount to 36,272, and the females to 33,128, or as
109.49 to 100. The males under seventeen years amounted to 10,773, and
the females under the same age to 9593, or as 112.3 to 100. From the
census of 1872, the proportion of males of all ages (including
half-castes) to females, is as 125.36 to 100. It must be borne in mind
that all these returns for the Sandwich Islands give the proportion of
living males to living females, and not of the births; and judging from
all civilised countries the proportion of males would have been
considerably higher if the numbers had referred to births. (99. Dr.
Coulter, in describing (‘Journal R. Geograph. Soc.’ vol. v. 1835, p.
67) the state of California about the year 1830, says that the natives,
reclaimed by the Spanish missionaries, have nearly all perished, or are
perishing, although well treated, not driven from their native land,
and kept from the use of spirits. He attributes this, in great part, to
the undoubted fact that the men greatly exceed the women in number; but
he does not know whether this is due to a failure of female offspring,
or to more females dying during early youth. The latter alternative,
according to all analogy, is very improbable. He adds that
“infanticide, properly so called, is not common, though very frequent
recourse is had to abortion.” If Dr. Coulter is correct about
infanticide, this case cannot be advanced in support of Colonel
Marshall’s view. From the rapid decrease of the reclaimed natives, we
may suspect that, as in the cases lately given, their fertility has
been diminished from changed habits of life.
I had hoped to gain some light on this subject from the breeding of
dogs; inasmuch as in most breeds, with the exception, perhaps, of
greyhounds, many more female puppies are destroyed than males, just as
with the Toda infants. Mr. Cupples assures me that this is usual with
Scotch deer-hounds. Unfortunately, I know nothing of the proportion of
the sexes in any breed, excepting greyhounds, and there the male births
are to the females as 110.1 to 100. Now from enquiries made from many
breeders, it seems that the females are in some respects more esteemed,
though otherwise troublesome; and it does not appear that the female
puppies of the best-bred dogs are systematically destroyed more than
the males, though this does sometimes take place to a limited extent.
Therefore I am unable to decide whether we can, on the above
principles, account for the preponderance of male births in greyhounds.
On the other hand, we have seen that with horses, cattle, and sheep,
which are too valuable for the young of either sex to be destroyed, if
there is any difference, the females are slightly in excess.)
From the several foregoing cases we have some reason to believe that
infanticide practised in the manner above explained, tends to make a
male-producing race; but I am far from supposing that this practice in
the case of man, or some analogous process with other species, has been
the sole determining cause of an excess of males. There may be some
unknown law leading to this result in decreasing races, which have
already become somewhat infertile. Besides the several causes
previously alluded to, the greater facility of parturition amongst
savages, and the less consequent injury to their male infants, would
tend to increase the proportion of live-born males to females. There
does not, however, seem to be any necessary connection between savage
life and a marked excess of males; that is if we may judge by the
character of the scanty offspring of the lately existing Tasmanians and
of the crossed offspring of the Tahitians now inhabiting Norfolk
Island.
As the males and females of many animals differ somewhat in habits and
are exposed in different degrees to danger, it is probable that in many
cases, more of one sex than of the other are habitually destroyed. But
as far as I can trace out the complication of causes, an indiscriminate
though large destruction of either sex would not tend to modify the
sex-producing power of the species. With strictly social animals, such
as bees or ants, which produce a vast number of sterile and fertile
females in comparison with the males, and to whom this preponderance is
of paramount importance, we can see that those communities would
flourish best which contained females having a strong inherited
tendency to produce more and more females; and in such cases an unequal
sex-producing tendency would be ultimately gained through natural
selection. With animals living in herds or troops, in which the males
come to the front and defend the herd, as with the bisons of North
America and certain baboons, it is conceivable that a male-producing
tendency might be gained by natural selection; for the individuals of
the better defended herds would leave more numerous descendants. In the
case of mankind the advantage arising from having a preponderance of
men in the tribe is supposed to be one chief cause of the practice of
female infanticide.
In no case, as far as we can see, would an inherited tendency to
produce both sexes in equal numbers or to produce one sex in excess, be
a direct advantage or disadvantage to certain individuals more than to
others; for instance, an individual with a tendency to produce more
males than females would not succeed better in the battle for life than
an individual with an opposite tendency; and therefore a tendency of
this kind could not be gained through natural selection. Nevertheless,
there are certain animals (for instance, fishes and cirripedes) in
which two or more males appear to be necessary for the fertilisation of
the female; and the males accordingly largely preponderate, but it is
by no means obvious how this male-producing tendency could have been
acquired. I formerly thought that when a tendency to produce the two
sexes in equal numbers was advantageous to the species, it would follow
from natural selection, but I now see that the whole problem is so
intricate that it is safer to leave its solution for the future.