The Descent of Man and Selection in Relation to Sex
by
Charles Darwin
Part 6 out of 17
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. 'Uber die knochernen Schadelhocker der
Vogel,' 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.
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 'Handworterbuch 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, 'Handworterbuch 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, 'Handworterbuch 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 Muller (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. Muller also observed that the
relative number of the two sexes in some species differed much in different
localities. But as H. Muller 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 Muller 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. 'Beitrage 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 Muller 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,
Muller 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 Muller 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.
CHAPTER IX.
SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES OF THE ANIMAL KINGDOM.
These characters absent in the lowest classes--Brilliant colours--Mollusca
--Annelids--Crustacea, secondary sexual characters strongly developed;
dimorphism; colour; characters not acquired before maturity--Spiders,
sexual colours of; stridulation by the males--Myriapoda.
With animals belonging to the lower classes, the two sexes are not rarely
united in the same individual, and therefore secondary sexual characters
cannot be developed. In many cases where the sexes are separate, both are
permanently attached to some support, and the one cannot search or struggle
for the other. Moreover it is almost certain that these animals have too
imperfect senses and much too low mental powers to appreciate each other's
beauty or other attractions, or to feel rivalry.
Hence in these classes or sub-kingdoms, such as the Protozoa, Coelenterata,
Echinodermata, Scolecida, secondary sexual characters, of the kind which we
have to consider, do not occur: and this fact agrees with the belief that
such characters in the higher classes have been acquired through sexual
selection, which depends on the will, desire, and choice of either sex.
Nevertheless some few apparent exceptions occur; thus, as I hear from Dr.
Baird, the males of certain Entozoa, or internal parasitic worms, differ
slightly in colour from the females; but we have no reason to suppose that
such differences have been augmented through sexual selection.
Contrivances by which the male holds the female, and which are
indispensable for the propagation of the species, are independent of sexual
selection, and have been acquired through ordinary selection.
Many of the lower animals, whether hermaphrodites or with separate sexes,
are ornamented with the most brilliant tints, or are shaded and striped in
an elegant manner; for instance, many corals and sea-anemones (Actiniae),
some jelly-fish (Medusae, Porpita, etc.), some Planariae, many star-fishes,
Echini, Ascidians, etc.; but we may conclude from the reasons already
indicated, namely, the union of the two sexes in some of these animals, the
permanently affixed condition of others, and the low mental powers of all,
that such colours do not serve as a sexual attraction, and have not been
acquired through sexual selection. It should be borne in mind that in no
case have we sufficient evidence that colours have been thus acquired,
except where one sex is much more brilliantly or conspicuously coloured
than the other, and where there is no difference in habits between the
sexes sufficient to account for their different colours. But the evidence
is rendered as complete as it can ever be, only when the more ornamented
individuals, almost always the males, voluntarily display their attractions
before the other sex; for we cannot believe that such display is useless,
and if it be advantageous, sexual selection will almost inevitably follow.
We may, however, extend this conclusion to both sexes, when coloured alike,
if their colours are plainly analogous to those of one sex alone in certain
other species of the same group.
How, then, are we to account for the beautiful or even gorgeous colours of
many animals in the lowest classes? It appears doubtful whether such
colours often serve as a protection; but that we may easily err on this
head, will be admitted by every one who reads Mr. Wallace's excellent essay
on this subject. It would not, for instance, at first occur to any one
that the transparency of the Medusae, or jelly-fish, is of the highest
service to them as a protection; but when we are reminded by Haeckel that
not only the Medusae, but many floating Mollusca, crustaceans, and even
small oceanic fishes partake of this same glass-like appearance, often
accompanied by prismatic colours, we can hardly doubt that they thus escape
the notice of pelagic birds and other enemies. M. Giard is also convinced
(1. 'Archives de Zoolog. Exper.' Oct. 1872, p. 563.) that the bright tints
of certain sponges and ascidians serve as a protection. Conspicuous
colours are likewise beneficial to many animals as a warning to their
would-be devourers that they are distasteful, or that they possess some
special means of defence; but this subject will be discussed more
conveniently hereafter.
We can, in our ignorance of most of the lowest animals, only say that their
bright tints result either from the chemical nature or the minute structure
of their tissues, independently of any benefit thus derived. Hardly any
colour is finer than that of arterial blood; but there is no reason to
suppose that the colour of the blood is in itself any advantage; and though
it adds to the beauty of the maiden's cheek, no one will pretend that it
has been acquired for this purpose. So again with many animals, especially
the lower ones, the bile is richly coloured; thus, as I am informed by Mr.
Hancock, the extreme beauty of the Eolidae (naked sea-slugs) is chiefly due
to the biliary glands being seen through the translucent integuments--this
beauty being probably of no service to these animals. The tints of the
decaying leaves in an American forest are described by every one as
gorgeous; yet no one supposes that these tints are of the least advantage
to the trees. Bearing in mind how many substances closely analogous to
natural organic compounds have been recently formed by chemists, and which
exhibit the most splendid colours, it would have been a strange fact if
substances similarly coloured had not often originated, independently of
any useful end thus gained, in the complex laboratory of living organisms.
THE SUB-KINGDOM OF THE MOLLUSCA.
Throughout this great division of the animal kingdom, as far as I can
discover, secondary sexual characters, such as we are here considering,
never occur. Nor could they be expected in the three lowest classes,
namely, in the Ascidians, Polyzoa, and Brachiopods (constituting the
Molluscoida of some authors), for most of these animals are permanently
affixed to a support or have their sexes united in the same individual. In
the Lamellibranchiata, or bivalve shells, hermaphroditism is not rare. In
the next higher class of the Gasteropoda, or univalve shells, the sexes are
either united or separate. But in the latter case the males never possess
special organs for finding, securing, or charming the females, or for
fighting with other males. As I am informed by Mr. Gwyn Jeffreys, the sole
external difference between the sexes consists in the shell sometimes
differing a little in form; for instance, the shell of the male periwinkle
(Littorina littorea) is narrower and has a more elongated spire than that
of the female. But differences of this nature, it may be presumed, are
directly connected with the act of reproduction, or with the development of
the ova.
The Gasteropoda, though capable of locomotion and furnished with imperfect
eyes, do not appear to be endowed with sufficient mental powers for the
members of the same sex to struggle together in rivalry, and thus to
acquire secondary sexual characters. Nevertheless with the pulmoniferous
gasteropods, or land-snails, the pairing is preceded by courtship; for
these animals, though hermaphrodites, are compelled by their structure to
pair together. Agassiz remarks, "Quiconque a eu l'occasion d'observer les
amours des limacons, ne saurait mettre en doute la seduction deployee dans
les mouvements et les allures qui preparent et accomplissent le double
embrassement de ces hermaphrodites." (2. 'De l'Espece et de la Class.'
etc., 1869, p. 106.) These animals appear also susceptible of some degree
of permanent attachment: an accurate observer, Mr. Lonsdale, informs me
that he placed a pair of land-snails, (Helix pomatia), one of which was
weakly, into a small and ill-provided garden. After a short time the
strong and healthy individual disappeared, and was traced by its track of
slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale
concluded that it had deserted its sickly mate; but after an absence of
twenty-four hours it returned, and apparently communicated the result of
its successful exploration, for both then started along the same track and
disappeared over the wall.
Even in the highest class of the Mollusca, the Cephalopoda or cuttle-
fishes, in which the sexes are separate, secondary sexual characters of the
present kind do not, as far as I can discover, occur. This is a surprising
circumstance, as these animals possess highly-developed sense-organs and
have considerable mental powers, as will be admitted by every one who has
watched their artful endeavours to escape from an enemy. (3. See, for
instance, the account which I have given in my 'Journal of Researches,'
1845, p. 7.) Certain Cephalopoda, however, are characterised by one
extraordinary sexual character, namely that the male element collects
within one of the arms or tentacles, which is then cast off, and clinging
by its sucking-discs to the female, lives for a time an independent life.
So completely does the cast-off arm resemble a separate animal, that it was
described by Cuvier as a parasitic worm under the name of Hectocotyle. But
this marvellous structure may be classed as a primary rather than as a
secondary sexual character.
Although with the Mollusca sexual selection does not seem to have come into
play; yet many univalve and bivalve shells, such as volutes, cones,
scallops, etc., are beautifully coloured and shaped. The colours do not
appear in most cases to be of any use as a protection; they are probably
the direct result, as in the lowest classes, of the nature of the tissues;
the patterns and the sculpture of the shell depending on its manner of
growth. The amount of light seems to be influential to a certain extent;
for although, as repeatedly stated by Mr. Gwyn Jeffreys, the shells of some
species living at a profound depth are brightly coloured, yet we generally
see the lower surfaces, as well as the parts covered by the mantle, less
highly-coloured than the upper and exposed surfaces. (4. I have given
('Geological Observations on Volcanic Islands,' 1844, p. 53) a curious
instance of the influence of light on the colours of a frondescent
incrustation, deposited by the surf on the coast-rocks of Ascension and
formed by the solution of triturated sea-shells.) In some cases, as with
shells living amongst corals or brightly-tinted seaweeds, the bright
colours may serve as a protection. (5. Dr. Morse has lately discussed
this subject in his paper on the 'Adaptive Coloration of Mollusca,' 'Proc.
Boston Soc. of Nat. Hist.' vol. xiv. April 1871.) But that many of the
nudibranch Mollusca, or sea-slugs, are as beautifully coloured as any
shells, may be seen in Messrs. Alder and Hancock's magnificent work; and
from information kindly given me by Mr. Hancock, it seems extremely
doubtful whether these colours usually serve as a protection. With some
species this may be the case, as with one kind which lives on the green
leaves of algae, and is itself bright-green. But many brightly-coloured,
white, or otherwise conspicuous species, do not seek concealment; whilst
again some equally conspicuous species, as well as other dull-coloured
kinds live under stones and in dark recesses. So that with these
nudibranch molluscs, colour apparently does not stand in any close relation
to the nature of the places which they inhabit.
These naked sea-slugs are hermaphrodites, yet they pair together, as do
land-snails, many of which have extremely pretty shells. It is conceivable
that two hermaphrodites, attracted by each other's greater beauty, might
unite and leave offspring which would inherit their parents' greater
beauty. But with such lowly-organised creatures this is extremely
improbable. Nor is it at all obvious how the offspring from the more
beautiful pairs of hermaphrodites would have any advantage over the
offspring of the less beautiful, so as to increase in number, unless indeed
vigour and beauty generally coincided. We have not here the case of a
number of males becoming mature before the females, with the more beautiful
males selected by the more vigorous females. If, indeed, brilliant colours
were beneficial to a hermaphrodite animal in relation to its general habits
of life, the more brightly-tinted individuals would succeed best and would
increase in number; but this would be a case of natural and not of sexual
selection.
SUB-KINGDOM OF THE VERMES: CLASS, ANNELIDA (OR SEA-WORMS).
In this class, although the sexes, when separate, sometimes differ from
each other in characters of such importance that they have been placed
under distinct genera or even families, yet the differences do not seem of
the kind which can be safely attributed to sexual selection. These animals
are often beautifully coloured, but as the sexes do not differ in this
respect, we are but little concerned with them. Even the Nemertians,
though so lowly organised, "vie in beauty and variety of colouring with any
other group in the invertebrate series"; yet Dr. McIntosh (6. See his
beautiful monograph on 'British Annelids,' part i. 1873, p. 3.) cannot
discover that these colours are of any service. The sedentary annelids
become duller-coloured, according to M. Quatrefages (7. See M. Perrier:
'L'Origine de l'Homme d'apres Darwin,' 'Revue Scientifique', Feb. 1873, p.
866.), after the period of reproduction; and this I presume may be
attributed to their less vigorous condition at that time. All these worm-
like animals apparently stand too low in the scale for the individuals of
either sex to exert any choice in selecting a partner, or for the
individuals of the same sex to struggle together in rivalry.
SUB-KINGDOM OF THE ARTHROPODA: CLASS, CRUSTACEA.
In this great class we first meet with undoubted secondary sexual
characters, often developed in a remarkable manner. Unfortunately the
habits of crustaceans are very imperfectly known, and we cannot explain the
uses of many structures peculiar to one sex. With the lower parasitic
species the males are of small size, and they alone are furnished with
perfect swimming-legs, antennae and sense-organs; the females being
destitute of these organs, with their bodies often consisting of a mere
distorted mass. But these extraordinary differences between the two sexes
are no doubt related to their widely different habits of life, and
consequently do not concern us. In various crustaceans, belonging to
distinct families, the anterior antennae are furnished with peculiar
thread-like bodies, which are believed to act as smelling-organs, and these
are much more numerous in the males than in the females. As the males,
without any unusual development of their olfactory organs, would almost
certainly be able sooner or later to find the females, the increased number
of the smelling-threads has probably been acquired through sexual
selection, by the better provided males having been the more successful in
finding partners and in producing offspring. Fritz Muller has described a
remarkable dimorphic species of Tanais, in which the male is represented by
two distinct forms, which never graduate into each other. In the one form
the male is furnished with more numerous smelling-threads, and in the other
form with more powerful and more elongated chelae or pincers, which serve
to hold the female. Fritz Muller suggests that these differences between
the two male forms of the same species may have originated in certain
individuals having varied in the number of the smelling-threads, whilst
other individuals varied in the shape and size of their chelae; so that of
the former, those which were best able to find the female, and of the
latter, those which were best able to hold her, have left the greatest
number of progeny to inherit their respective advantages. (8. 'Facts and
Arguments for Darwin,' English translat., 1869, p. 20. See the previous
discussion on the olfactory threads. Sars has described a somewhat
analogous case (as quoted in 'Nature,' 1870, p. 455) in a Norwegian
crustacean, the Pontoporeia affinis.)
[Fig.4. Labidocera Darwinii (from Lubbock). Labelled are:
a. Part of right anterior antenna of male, forming a prehensile organ.
b. Posterior pair of thoracic legs of male.
c. Ditto of female.]
In some of the lower crustaceans, the right anterior antenna of the male
differs greatly in structure from the left, the latter resembling in its
simple tapering joints the antennae of the female. In the male the
modified antenna is either swollen in the middle or angularly bent, or
converted (Fig. 4) into an elegant, and sometimes wonderfully complex,
prehensile organ. (9. See Sir J. Lubbock in 'Annals and Mag. of Nat.
Hist.' vol. xi. 1853, pl. i. and x.; and vol. xii. (1853), pl. vii. See
also Lubbock in 'Transactions, Entomological Society,' vol. iv. new series,
1856-1858, p. 8. With respect to the zigzagged antennae mentioned below,
see Fritz Muller, 'Facts and Arguments for Darwin,' 1869, p. 40, foot-
note.) It serves, as I hear from Sir J. Lubbock, to hold the female, and
for this same purpose one of the two posterior legs (b) on the same side of
the body is converted into a forceps. In another family the inferior or
posterior antennae are "curiously zigzagged" in the males alone.
[Fig. 5. Anterior part of body of Callianassa (from Milne-Edwards),
showing the unequal and differently-constructed right and left-hand chelae
of the male. N.B.--The artist by mistake has reversed the drawing, and
made the left-hand chela the largest.
Fig. 6. Second leg of male Orchestia Tucuratinga (from Fritz Muller).
Fig. 7. Ditto of female.]
In the higher crustaceans the anterior legs are developed into chelae or
pincers; and these are generally larger in the male than in the female,--so
much so that the market value of the male edible crab (Cancer pagurus),
according to Mr. C. Spence Bate, is five times as great as that of the
female. In many species the chelae are of unequal size on the opposite
side of the body, the right-hand one being, as I am informed by Mr. Bate,
generally, though not invariably, the largest. This inequality is also
often much greater in the male than in the female. The two chelae of the
male often differ in structure (Figs. 5, 6, and 7), the smaller one
resembling that of the female. What advantage is gained by their
inequality in size on the opposite sides of the body, and by the inequality
being much greater in the male than in the female; and why, when they are
of equal size, both are often much larger in the male than in the female,
is not known. As I hear from Mr. Bate, the chelae are sometimes of such
length and size that they cannot possibly be used for carrying food to the
mouth. In the males of certain fresh-water prawns (Palaemon) the right leg
is actually longer than the whole body. (10. See a paper by Mr. C. Spence
Bate, with figures, in 'Proceedings, Zoological Society,' 1868, p. 363; and
on the nomenclature of the genus, ibid. p. 585. I am greatly indebted to
Mr. Spence Bate for nearly all the above statements with respect to the
chelae of the higher crustaceans.) The great size of the one leg with its
chelae may aid the male in fighting with his rivals; but this will not
account for their inequality in the female on the opposite sides of the
body. In Gelasimus, according to a statement quoted by Milne Edwards (11.
'Hist. Nat. des Crust.' tom. ii. 1837, p. 50.), the male and the female
live in the same burrow, and this shews that they pair; the male closes the
mouth of the burrow with one of its chelae, which is enormously developed;
so that here it indirectly serves as a means of defence. Their main use,
however, is probably to seize and to secure the female, and this in some
instances, as with Gammarus, is known to be the case. The male of the
hermit or soldier crab (Pagurus) for weeks together, carries about the
shell inhabited by the female. (12. Mr. C. Spence Bate, 'British
Association, Fourth Report on the Fauna of S. Devon.') The sexes, however,
of the common shore-crab (Carcinus maenas), as Mr. Bate informs me, unite
directly after the female has moulted her hard shell, when she is so soft
that she would be injured if seized by the strong pincers of the male; but
as she is caught and carried about by the male before moulting, she could
then be seized with impunity.
[Fig.8. Orchestia Darwinii (from Fritz Muller), showing the differently-
constructed chelae of the two male forms.]
Fritz Muller states that certain species of Melita are distinguished from
all other amphipods by the females having "the coxal lamellae of the
penultimate pair of feet produced into hook-like processes, of which the
males lay hold with the hands of the first pair." The development of these
hook-like processes has probably followed from those females which were the
most securely held during the act of reproduction, having left the largest
number of offspring. Another Brazilian amphipod (see Orchestia darwinii,
Fig. 8) presents a case of dimorphism, like that of Tanais; for there are
two male forms, which differ in the structure of their chelae. (13. Fritz
Muller, 'Facts and Arguments for Darwin,' 1869, pp. 25-28.) As either
chela would certainly suffice to hold the female,--for both are now used
for this purpose,--the two male forms probably originated by some having
varied in one manner and some in another; both forms having derived certain
special, but nearly equal advantages, from their differently shaped organs.
It is not known that male crustaceans fight together for the possession of
the females, but it is probably the case; for with most animals when the
male is larger than the female, he seems to owe his greater size to his
ancestors having fought with other males during many generations. In most
of the orders, especially in the highest or the Brachyura, the male is
larger than the female; the parasitic genera, however, in which the sexes
follow different habits of life, and most of the Entomostraca must be
excepted. The chelae of many crustaceans are weapons well adapted for
fighting. Thus when a Devil-crab (Portunus puber) was seen by a son of Mr.
Bate fighting with a Carcinus maenas, the latter was soon thrown on its
back, and had every limb torn from its body. When several males of a
Brazilian Gelasimus, a species furnished with immense pincers, were placed
together in a glass vessel by Fritz Muller, they mutilated and killed one
another. Mr. Bate put a large male Carcinus maenas into a pan of water,
inhabited by a female which was paired with a smaller male; but the latter
was soon dispossessed. Mr. Bate adds, "if they fought, the victory was a
bloodless one, for I saw no wounds." This same naturalist separated a male
sand-skipper (so common on our sea-shores), Gammarus marinus, from its
female, both of whom were imprisoned in the same vessel with many
individuals of the same species. The female, when thus divorced, soon
joined the others. After a time the male was put again into the same
vessel; and he then, after swimming about for a time, dashed into the
crowd, and without any fighting at once took away his wife. This fact
shews that in the Amphipoda, an order low in the scale, the males and
females recognise each other, and are mutually attached.
The mental powers of the Crustacea are probably higher than at first sight
appears probable. Any one who tries to catch one of the shore-crabs, so
common on tropical coasts, will perceive how wary and alert they are.
There is a large crab (Birgus latro), found on coral islands, which makes a
thick bed of the picked fibres of the cocoa-nut, at the bottom of a deep
burrow. It feeds on the fallen fruit of this tree by tearing off the husk,
fibre by fibre; and it always begins at that end where the three eye-like
depressions are situated. It then breaks through one of these eyes by
hammering with its heavy front pincers, and turning round, extracts the
albuminous core with its narrow posterior pincers. But these actions are
probably instinctive, so that they would be performed as well by a young
animal as by an old one. The following case, however, can hardly be so
considered: a trustworthy naturalist, Mr. Gardner (14. 'Travels in the
Interior of Brazil,' 1846, p. 111. I have given, in my 'Journal of
Researches,' p. 463, an account of the habits of the Birgus.), whilst
watching a shore-crab (Gelasimus) making its burrow, threw some shells
towards the hole. One rolled in, and three other shells remained within a
few inches of the mouth. In about five minutes the crab brought out the
shell which had fallen in, and carried it away to a distance of a foot; it
then saw the three other shells lying near, and evidently thinking that
they might likewise roll in, carried them to the spot where it had laid the
first. It would, I think, be difficult to distinguish this act from one
performed by man by the aid of reason.
Mr. Bate does not know of any well-marked case of difference of colour in
the two sexes of our British crustaceans, in which respect the sexes of the
higher animals so often differ. In some cases, however, the males and
females differ slightly in tint, but Mr. Bate thinks not more than may be
accounted for by their different habits of life, such as by the male
wandering more about, and being thus more exposed to the light. Dr. Power
tried to distinguish by colour the sexes of the several species which
inhabit the Mauritius, but failed, except with one species of Squilla,
probably S. stylifera, the male of which is described as being "of a
beautiful bluish-green," with some of the appendages cherry-red, whilst the
female is clouded with brown and grey, "with the red about her much less
vivid than in the male." (15. Mr. Ch. Fraser, in 'Proc. Zoolog. Soc.'
1869, p. 3. I am indebted to Mr. Bate for Dr. Power's statement.) In this
case, we may suspect the agency of sexual selection. From M. Bert's
observations on Daphnia, when placed in a vessel illuminated by a prism, we
have reason to believe that even the lowest crustaceans can distinguish
colours. With Saphirina (an oceanic genus of Entomostraca), the males are
furnished with minute shields or cell-like bodies, which exhibit beautiful
changing colours; these are absent in the females, and in both sexes of one
species. (16. Claus, 'Die freilebenden Copepoden,' 1863, s. 35.) It
would, however, be extremely rash to conclude that these curious organs
serve to attract the females. I am informed by Fritz Muller, that in the
female of a Brazilian species of Gelasimus, the whole body is of a nearly
uniform greyish-brown. In the male the posterior part of the cephalo-
thorax is pure white, with the anterior part of a rich green, shading into
dark brown; and it is remarkable that these colours are liable to change in
the course of a few minutes--the white becoming dirty grey or even black,
the green "losing much of its brilliancy." It deserves especial notice
that the males do not acquire their bright colours until they become
mature. They appear to be much more numerous than the females; they differ
also in the larger size of their chelae. In some species of the genus,
probably in all, the sexes pair and inhabit the same burrow. They are
also, as we have seen, highly intelligent animals. From these various
considerations it seems probable that the male in this species has become
gaily ornamented in order to attract or excite the female.
It has just been stated that the male Gelasimus does not acquire his
conspicuous colours until mature and nearly ready to breed. This seems a
general rule in the whole class in respect to the many remarkable
structural differences between the sexes. We shall hereafter find the same
law prevailing throughout the great sub-kingdom of the Vertebrata; and in
all cases it is eminently distinctive of characters which have been
acquired through sexual selection. Fritz Muller (17. 'Facts and
Arguments,' etc., p. 79.) gives some striking instances of this law; thus
the male sand-hopper (Orchestia) does not, until nearly full grown, acquire
his large claspers, which are very differently constructed from those of
the female; whilst young, his claspers resemble those of the female.
CLASS, ARACHNIDA (SPIDERS).
The sexes do not generally differ much in colour, but the males are often
darker than the females, as may be seen in Mr. Blackwall's magnificent
work. (18. 'A History of the Spiders of Great Britain,' 1861-64. For the
following facts, see pp. 77, 88, 102.) In some species, however, the
difference is conspicuous: thus the female of Sparassus smaragdulus is
dullish green, whilst the adult male has the abdomen of a fine yellow, with
three longitudinal stripes of rich red. In certain species of Thomisus the
sexes closely resemble each other, in others they differ much; and
analogous cases occur in many other genera. It is often difficult to say
which of the two sexes departs most from the ordinary coloration of the
genus to which the species belong; but Mr. Blackwall thinks that, as a
general rule, it is the male; and Canestrini (19. This author has recently
published a valuable essay on the 'Caratteri sessuali secondarii degli
Arachnidi,' in the 'Atti della Soc. Veneto-Trentina di Sc. Nat. Padova,'
vol. i. Fasc. 3, 1873.) remarks that in certain genera the males can be
specifically distinguished with ease, but the females with great
difficulty. I am informed by Mr. Blackwall that the sexes whilst young
usually resemble each other; and both often undergo great changes in colour
during their successive moults, before arriving at maturity. In other
cases the male alone appears to change colour. Thus the male of the above
bright-coloured Sparassus at first resembles the female, and acquires his
peculiar tints only when nearly adult. Spiders are possessed of acute
senses, and exhibit much intelligence; as is well known, the females often
shew the strongest affection for their eggs, which they carry about
enveloped in a silken web. The males search eagerly for the females, and
have been seen by Canestrini and others to fight for possession of them.
This same author says that the union of the two sexes has been observed in
about twenty species; and he asserts positively that the female rejects
some of the males who court her, threatens them with open mandibles, and at
last after long hesitation accepts the chosen one. From these several
considerations, we may admit with some confidence that the well-marked
differences in colour between the sexes of certain species are the results
of sexual selection; though we have not here the best kind of evidence,--
the display by the male of his ornaments. From the extreme variability of
colour in the male of some species, for instance of Theridion lineatum, it
would appear that these sexual characters of the males have not as yet
become well fixed. Canestrini draws the same conclusion from the fact that
the males of certain species present two forms, differing from each other
in the size and length of their jaws; and this reminds us of the above
cases of dimorphic crustaceans.
The male is generally much smaller than the female, sometimes to an
extraordinary degree (20. Aug. Vinson ('Araneides des Iles de la Reunion,'
pl. vi. figs. 1 and 2) gives a good instance of the small size of the male,
in Epeira nigra. In this species, as I may add, the male is testaceous and
the female black with legs banded with red. Other even more striking cases
of inequality in size between the sexes have been recorded ('Quarterly
Journal of Science,' July 1868, p. 429); but I have not seen the original
accounts.), and he is forced to be extremely cautious in making his
advances, as the female often carries her coyness to a dangerous pitch. De
Geer saw a male that "in the midst of his preparatory caresses was seized
by the object of his attentions, enveloped by her in a web and then
devoured, a sight which, as he adds, filled him with horror and
indignation." (21. Kirby and Spence, 'Introduction to Entomology,' vol.
i. 1818, p. 280.) The Rev. O.P. Cambridge (22. 'Proceedings, Zoological
Society,' 1871, p. 621.) accounts in the following manner for the extreme
smallness of the male in the genus Nephila. "M. Vinson gives a graphic
account of the agile way in which the diminutive male escapes from the
ferocity of the female, by gliding about and playing hide and seek over her
body and along her gigantic limbs: in such a pursuit it is evident that
the chances of escape would be in favour of the smallest males, while the
larger ones would fall early victims; thus gradually a diminutive race of
males would be selected, until at last they would dwindle to the smallest
possible size compatible with the exercise of their generative functions,--
in fact, probably to the size we now see them, i.e., so small as to be a
sort of parasite upon the female, and either beneath her notice, or too
agile and too small for her to catch without great difficulty."
Westring has made the interesting discovery that the males of several
species of Theridion (23. Theridion (Asagena, Sund.) serratipes, 4-
punctatum et guttatum; see Westring, in Kroyer, 'Naturhist. Tidskrift,'
vol. iv. 1842-1843, p. 349; and vol. ii. 1846-1849, p. 342. See, also, for
other species, 'Araneae Suecicae,' p. 184.) have the power of making a
stridulating sound, whilst the females are mute. The apparatus consists of
a serrated ridge at the base of the abdomen, against which the hard hinder
part of the thorax is rubbed; and of this structure not a trace can be
detected in the females. It deserves notice that several writers,
including the well-known arachnologist Walckenaer, have declared that
spiders are attracted by music. (24. Dr. H.H. van Zouteveen, in his Dutch
translation of this work (vol. i. p. 444), has collected several cases.)
From the analogy of the Orthoptera and Homoptera, to be described in the
next chapter, we may feel almost sure that the stridulation serves, as
Westring also believes, to call or to excite the female; and this is the
first case known to me in the ascending scale of the animal kingdom of
sounds emitted for this purpose. (25. Hilgendorf, however, has lately
called attention to an analogous structure in some of the higher
crustaceans, which seems adapted to produce sound; see 'Zoological Record,'
1869, p. 603.)
CLASS, MYRIAPODA.
In neither of the two orders in this class, the millipedes and centipedes,
can I find any well-marked instances of such sexual differences as more
particularly concern us. In Glomeris limbata, however, and perhaps in some
few other species, the males differ slightly in colour from the females;
but this Glomeris is a highly variable species. In the males of the
Diplopoda, the legs belonging either to one of the anterior or of the
posterior segments of the body are modified into prehensile hooks which
serve to secure the female. In some species of Iulus the tarsi of the male
are furnished with membranous suckers for the same purpose. As we shall
see when we treat of Insects, it is a much more unusual circumstance, that
it is the female in Lithobius, which is furnished with prehensile
appendages at the extremity of her body for holding the male. (26.
Walckenaer et P. Gervais, 'Hist. Nat. des Insectes: Apteres,' tom. iv.
1847, pp. 17, 19, 68.)
CHAPTER X.
SECONDARY SEXUAL CHARACTERS OF INSECTS.
Diversified structures possessed by the males for seizing the females--
Differences between the sexes, of which the meaning is not understood--
Difference in size between the sexes--Thysanura--Diptera--Hemiptera--
Homoptera, musical powers possessed by the males alone--Orthoptera, musical
instruments of the males, much diversified in structure; pugnacity;
colours--Neuroptera, sexual differences in colour--Hymenoptera, pugnacity
and odours--Coleoptera, colours; furnished with great horns, apparently as
an ornament; battles, stridulating organs generally common to both sexes.
In the immense class of insects the sexes sometimes differ in their
locomotive-organs, and often in their sense-organs, as in the pectinated
and beautifully plumose antennae of the males of many species. In Chloeon,
one of the Ephemerae, the male has great pillared eyes, of which the female
is entirely destitute. (1. Sir J. Lubbock, 'Transact. Linnean Soc.' vol.
xxv, 1866, p. 484. With respect to the Mutillidae see Westwood, 'Modern
Class. of Insects,' vol. ii. p. 213.) The ocelli are absent in the females
of certain insects, as in the Mutillidae; and here the females are likewise
wingless. But we are chiefly concerned with structures by which one male
is enabled to conquer another, either in battle or courtship, through his
strength, pugnacity, ornaments, or music. The innumerable contrivances,
therefore, by which the male is able to seize the female, may be briefly
passed over. Besides the complex structures at the apex of the abdomen,
which ought perhaps to be ranked as primary organs (2. These organs in the
male often differ in closely-allied species, and afford excellent specific
characters. But their importance, from a functional point of view, as Mr.
R. MacLachlan has remarked to me, has probably been overrated. It has been
suggested, that slight differences in these organs would suffice to prevent
the intercrossing of well-marked varieties or incipient species, and would
thus aid in their development. That this can hardly be the case, we may
infer from the many recorded cases (see, for instance, Bronn, 'Geschichte
der Natur,' B. ii. 1843, s. 164; and Westwood, 'Transact. Ent. Soc.' vol.
iii. 1842, p. 195) of distinct species having been observed in union. Mr.
MacLachlan informs me (vide 'Stett. Ent. Zeitung,' 1867, s. 155) that when
several species of Phryganidae, which present strongly-pronounced
differences of this kind, were confined together by Dr. Aug. Meyer, THEY
COUPLED, and one pair produced fertile ova.), "it is astonishing," as Mr.
B.D. Walsh (3. 'The Practical Entomologist,' Philadelphia, vol. ii. May
1867, p 88.) has remarked, "how many different organs are worked in by
nature for the seemingly insignificant object of enabling the male to grasp
the female firmly." The mandibles or jaws are sometimes used for this
purpose; thus the male Corydalis cornutus (a neuropterous insect in some
degree allied to the Dragon flies, etc.) has immense curved jaws, many
times longer than those of the female; and they are smooth instead of being
toothed, so that he is thus enabled to seize her without injury. (4. Mr.
Walsh, ibid. p. 107.) One of the stag-beetles of North America (Lucanus
elaphus) uses his jaws, which are much larger than those of the female, for
the same purpose, but probably likewise for fighting. In one of the sand-
wasps (Ammophila) the jaws in the two sexes are closely alike, but are used
for widely different purposes: the males, as Professor Westwood observes,
"are exceedingly ardent, seizing their partners round the neck with their
sickle-shaped jaws" (5. 'Modern Classification of Insects,' vol. ii. 1840,
pp. 205, 206. Mr. Walsh, who called my attention to the double use of the
jaws, says that he has repeatedly observed this fact.); whilst the females
use these organs for burrowing in sand-banks and making their nests.
[Fig. 9. Crabro cribrarius. Upper figure, male; lower figure, female.]
The tarsi of the front-legs are dilated in many male beetles, or are
furnished with broad cushions of hairs; and in many genera of water-beetles
they are armed with a round flat sucker, so that the male may adhere to the
slippery body of the female. It is a much more unusual circumstance that
the females of some water-beetles (Dytiscus) have their elytra deeply
grooved, and in Acilius sulcatus thickly set with hairs, as an aid to the
male. The females of some other water-beetles (Hydroporus) have their
elytra punctured for the same purpose. (6. We have here a curious and
inexplicable case of dimorphism, for some of the females of four European
species of Dytiscus, and of certain species of Hydroporus, have their
elytra smooth; and no intermediate gradations between the sulcated or
punctured, and the quite smooth elytra have been observed. See Dr. H.
Schaum, as quoted in the 'Zoologist,' vols. v.-vi. 1847-48, p. 1896. Also
Kirby and Spence, 'Introduction to Entomology,' vol. iii. 1826, p. 305.)
In the male of Crabro cribrarius (Fig. 9), it is the tibia which is dilated
into a broad horny plate, with minute membraneous dots, giving to it a
singular appearance like that of a riddle. (7. Westwood, 'Modern Class.'
vol. ii. p. 193. The following statement about Penthe, and others in
inverted commas, are taken from Mr. Walsh, 'Practical Entomologist,'
Philadelphia, vol. iii. p. 88.) In the male of Penthe (a genus of beetles)
a few of the middle joints of the antennae are dilated and furnished on the
inferior surface with cushions of hair, exactly like those on the tarsi of
the Carabidae, "and obviously for the same end." In male dragon-flies,
"the appendages at the tip of the tail are modified in an almost infinite
variety of curious patterns to enable them to embrace the neck of the
female." Lastly, in the males of many insects, the legs are furnished with
peculiar spines, knobs or spurs; or the whole leg is bowed or thickened,
but this is by no means invariably a sexual character; or one pair, or all
three pairs are elongated, sometimes to an extravagant length. (8. Kirby
and Spence, 'Introduct.' etc., vol. iii. pp. 332-336.)
[Fig. 10. Taphroderes distortus (much enlarged). Upper figure, male;
lower figure, female.]
The sexes of many species in all the orders present differences, of which
the meaning is not understood. One curious case is that of a beetle (Fig.
10), the male of which has left mandible much enlarged; so that the mouth
is greatly distorted. In another Carabidous beetle, Eurygnathus (9.
'Insecta Maderensia,' 1854, page 20.), we have the case, unique as far as
known to Mr. Wollaston, of the head of the female being much broader and
larger, though in a variable degree, than that of the male. Any number of
such cases could be given. They abound in the Lepidoptera: one of the
most extraordinary is that certain male butterflies have their fore-legs
more or less atrophied, with the tibiae and tarsi reduced to mere
rudimentary knobs. The wings, also, in the two sexes often differ in
neuration (10. E. Doubleday, 'Annals and Mag. of Nat. Hist.' vol. i. 1848,
p. 379. I may add that the wings in certain Hymenoptera (see Shuckard,
'Fossorial Hymenoptera,' 1837, pp. 39-43) differ in neuration according to
sex.), and sometimes considerably in outline, as in the Aricoris epitus,
which was shewn to me in the British Museum by Mr. A. Butler. The males of
certain South American butterflies have tufts of hair on the margins of the
wings, and horny excrescences on the discs of the posterior pair. (11.
H.W. Bates, in 'Journal of Proc. Linn. Soc.' vol. vi. 1862, p. 74. Mr.
Wonfor's observations are quoted in 'Popular Science Review,' 1868, p.
343.) In several British butterflies, as shewn by Mr. Wonfor, the males
alone are in parts clothed with peculiar scales.
The use of the bright light of the female glow-worm has been subject to
much discussion. The male is feebly luminous, as are the larvae and even
the eggs. It has been supposed by some authors that the light serves to
frighten away enemies, and by others to guide the male to the female. At
last, Mr. Belt (12. 'The Naturalist in Nicaragua,' 1874, pp. 316-320. On
the phosphorescence of the eggs, see 'Annals and Magazine of Natural
History,' Nov. 1871, p. 372.) appears to have solved the difficulty: he
finds that all the Lampyridae which he has tried are highly distasteful to
insectivorous mammals and birds. Hence it is in accordance with Mr. Bates'
view, hereafter to be explained, that many insects mimic the Lampyridae
closely, in order to be mistaken for them, and thus to escape destruction.
He further believes that the luminous species profit by being at once
recognised as unpalatable. It is probable that the same explanation may be
extended to the Elaters, both sexes of which are highly luminous. It is
not known why the wings of the female glow-worm have not been developed;
but in her present state she closely resembles a larva, and as larvae are
so largely preyed on by many animals, we can understand why she has been
rendered so much more luminous and conspicuous than the male; and why the
larvae themselves are likewise luminous.
DIFFERENCE IN SIZE BETWEEN THE SEXES.
With insects of all kinds the males are commonly smaller than the females;
and this difference can often be detected even in the larval state. So
considerable is the difference between the male and female cocoons of the
silk-moth (Bombyx mori), that in France they are separated by a particular
mode of weighing. (13. Robinet, 'Vers a Soie,' 1848, p. 207.) In the
lower classes of the animal kingdom, the greater size of the females seems
generally to depend on their developing an enormous number of ova; and this
may to a certain extent hold good with insects. But Dr. Wallace has
suggested a much more probable explanation. He finds, after carefully
attending to the development of the caterpillars of Bombyx cynthia and
yamamai, and especially to that of some dwarfed caterpillars reared from a
second brood on unnatural food, "that in proportion as the individual moth
is finer, so is the time required for its metamorphosis longer; and for
this reason the female, which is the larger and heavier insect, from having
to carry her numerous eggs, will be preceded by the male, which is smaller
and has less to mature." (14. 'Transact. Ent. Soc.' 3rd series, vol. v.
p. 486.) Now as most insects are short-lived, and as they are exposed to
many dangers, it would manifestly be advantageous to the female to be
impregnated as soon as possible. This end would be gained by the males
being first matured in large numbers ready for the advent of the females;
and this again would naturally follow, as Mr. A.R. Wallace has remarked
(15. 'Journal of Proc. Ent. Soc.' Feb. 4, 1867, p. lxxi.), through natural
selection; for the smaller males would be first matured, and thus would
procreate a large number of offspring which would inherit the reduced size
of their male parents, whilst the larger males from being matured later
would leave fewer offspring.
There are, however, exceptions to the rule of male insects being smaller
than the females: and some of these exceptions are intelligible. Size and
strength would be an advantage to the males, which fight for the possession
of the females; and in these cases, as with the stag-beetle (Lucanus), the
males are larger than the females. There are, however, other beetles which
are not known to fight together, of which the males exceed the females in
size; and the meaning of this fact is not known; but in some of these
cases, as with the huge Dynastes and Megasoma, we can at least see that
there would be no necessity for the males to be smaller than the females,
in order to be matured before them, for these beetles are not short-lived,
and there would be ample time for the pairing of the sexes. So again, male
dragon-flies (Libellulidae) are sometimes sensibly larger, and never
smaller, than the females (16. For this and other statements on the size
of the sexes, see Kirby and Spence, ibid. vol. iii. p. 300; on the duration
of life in insects, see p. 344.); and as Mr. MacLachlan believes, they do
not generally pair with the females until a week or fortnight has elapsed,
and until they have assumed their proper masculine colours. But the most
curious case, shewing on what complex and easily-overlooked relations, so
trifling a character as difference in size between the sexes may depend, is
that of the aculeate Hymenoptera; for Mr. F. Smith informs me that
throughout nearly the whole of this large group, the males, in accordance
with the general rule, are smaller than the females, and emerge about a
week before them; but amongst the Bees, the males of Apis mellifica,
Anthidium manicatum, and Anthophora acervorum, and amongst the Fossores,
the males of the Methoca ichneumonides, are larger than the females. The
explanation of this anomaly is that a marriage flight is absolutely
necessary with these species, and the male requires great strength and size
in order to carry the female through the air. Increased size has here been
acquired in opposition to the usual relation between size and the period of
development, for the males, though larger, emerge before the smaller
females.
We will now review the several Orders, selecting such facts as more
particularly concern us. The Lepidoptera (Butterflies and Moths) will be
retained for a separate chapter.
ORDER, THYSANURA.
The members of this lowly organised order are wingless, dull-coloured,
minute insects, with ugly, almost misshapen heads and bodies. Their sexes
do not differ, but they are interesting as shewing us that the males pay
sedulous court to the females even low down in the animal scale. Sir J.
Lubbock (17. 'Transact. Linnean Soc.' vol. xxvi. 1868, p. 296.) says: "it
is very amusing to see these little creatures (Smynthurus luteus)
coquetting together. The male, which is much smaller than the female, runs
round her, and they butt one another, standing face to face and moving
backward and forward like two playful lambs. Then the female pretends to
run away and the male runs after her with a queer appearance of anger, gets
in front and stands facing her again; then she turns coyly round, but he,
quicker and more active, scuttles round too, and seems to whip her with his
antennae; then for a bit they stand face to face, play with their antennae,
and seem to be all in all to one another."
ORDER, DIPTERA (FLIES).
The sexes differ little in colour. The greatest difference, known to Mr.
F. Walker, is in the genus Bibio, in which the males are blackish or quite
black, and the females obscure brownish-orange. The genus Elaphomyia,
discovered by Mr. Wallace (18. 'The Malay Archipelago,' vol. ii. 1869, p.
313.) in New Guinea, is highly remarkable, as the males are furnished with
horns, of which the females are quite destitute. The horns spring from
beneath the eyes, and curiously resemble those of a stag, being either
branched or palmated. In one of the species, they equal the whole body in
length. They might be thought to be adapted for fighting, but as in one
species they are of a beautiful pink colour, edged with black, with a pale
central stripe, and as these insects have altogether a very elegant
appearance, it is perhaps more probable that they serve as ornaments. That
the males of some Diptera fight together is certain; Prof. Westwood (19.
'Modern Classification of Insects,' vol. ii. 1840, p. 526.) has several
times seen this with the Tipulae. The males of other Diptera apparently
try to win the females by their music: H. Muller (20. 'Anwendung,' etc.,
'Verh. d. n. V. Jahrg.' xxix. p. 80. Mayer, in 'American Naturalist,'
1874, p. 236.) watched for some time two males of an Eristalis courting a
female; they hovered above her, and flew from side to side, making a high
humming noise at the same time. Gnats and mosquitoes (Culicidae) also seem
to attract each other by humming; and Prof. Mayer has recently ascertained
that the hairs on the antennae of the male vibrate in unison with the notes
of a tuning-fork, within the range of the sounds emitted by the female.
The longer hairs vibrate sympathetically with the graver notes, and the
shorter hairs with the higher ones. Landois also asserts that he has
repeatedly drawn down a whole swarm of gnats by uttering a particular note.
It may be added that the mental faculties of the Diptera are probably
higher than in most other insects, in accordance with their highly-
developed nervous system. (21. See Mr. B.T. Lowne's interesting work, 'On
the Anatomy of the Blow-fly, Musca vomitoria,' 1870, p. 14. He remarks (p.
33) that, "the captured flies utter a peculiar plaintive note, and that
this sound causes other flies to disappear.")
ORDER, HEMIPTERA (FIELD-BUGS).
Mr. J.W. Douglas, who has particularly attended to the British species, has
kindly given me an account of their sexual differences. The males of some
species are furnished with wings, whilst the females are wingless; the
sexes differ in the form of their bodies, elytra, antennae and tarsi; but
as the signification of these differences are unknown, they may be here
passed over. The females are generally larger and more robust than the
males. With British, and, as far as Mr. Douglas knows, with exotic
species, the sexes do not commonly differ much in colour; but in about six
British species the male is considerably darker than the female, and in
about four other species the female is darker than the male. Both sexes of
some species are beautifully coloured; and as these insects emit an
extremely nauseous odour, their conspicuous colours may serve as a signal
that they are unpalatable to insectivorous animals. In some few cases
their colours appear to be directly protective: thus Prof. Hoffmann
informs me that he could hardly distinguish a small pink and green species
from the buds on the trunks of lime-trees, which this insect frequents.
Some species of Reduvidae make a stridulating noise; and, in the case of
Pirates stridulus, this is said (22. Westwood, 'Modern Classification of
Insects,' vol. ii. p. 473.) to be effected by the movement of the neck
within the pro-thoracic cavity. According to Westring, Reduvius personatus
also stridulates. But I have no reason to suppose that this is a sexual
character, excepting that with non-social insects there seems to be no use
for sound-producing organs, unless it be as a sexual call.
ORDER: HOMOPTERA.
Every one who has wandered in a tropical forest must have been astonished
at the din made by the male Cicadae. The females are mute; as the Grecian
poet Xenarchus says, "Happy the Cicadas live, since they all have voiceless
wives." The noise thus made could be plainly heard on board the "Beagle,"
when anchored at a quarter of a mile from the shore of Brazil; and Captain
Hancock says it can be heard at the distance of a mile. The Greeks
formerly kept, and the Chinese now keep these insects in cages for the sake
of their song, so that it must be pleasing to the ears of some men. (23.
These particulars are taken from Westwood's 'Modern Classification of
Insects,' vol. ii. 1840, p. 422. See, also, on the Fulgoridae, Kirby and
Spence, 'Introduct.' vol. ii. p. 401.) The Cicadidae usually sing during
the day, whilst the Fulgoridae appear to be night-songsters. The sound,
according to Landois (24. 'Zeitschrift fur wissenschaft Zoolog.' B. xvii.
1867, ss. 152-158.), is produced by the vibration of the lips of the
spiracles, which are set into motion by a current of air emitted from the
tracheae; but this view has lately been disputed. Dr. Powell appears to
have proved (25. 'Transactions of the New Zealand Institute,' vol. v.
1873, p. 286.) that it is produced by the vibration of a membrane, set into
action by a special muscle. In the living insect, whilst stridulating,
this membrane can be seen to vibrate; and in the dead insect the proper
sound is heard, if the muscle, when a little dried and hardened, is pulled
with the point of a pin. In the female the whole complex musical apparatus
is present, but is much less developed than in the male, and is never used
for producing sound.
With respect to the object of the music, Dr. Hartman, in speaking of the
Cicada septemdecim of the United States, says (26. I am indebted to Mr.
Walsh for having sent me this extract from 'A Journal of the Doings of
Cicada septemdecim,' by Dr. Hartman.), "the drums are now (June 6th and
7th, 1851) heard in all directions. This I believe to be the marital
summons from the males. Standing in thick chestnut sprouts about as high
as my head, where hundreds were around me, I observed the females coming
around the drumming males." He adds, "this season (Aug. 1868) a dwarf
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