Encyclopaedia Britannica > Volume 21, ROT-Siam

SEX
possess appendages modified for attachment, and sometimes retain
See ARicHNiDA 6 “ dlstinguished from the parasitic females.
Among Insects the sexes are distinguished by varying modifica
tions of different parts of the body, and differences in general form
and in colour are frequent. The males are generally active a
more beautiful, and seem better endowed with sense organs, though
usually smaller than the females. The males have also a me
eminence or even monopoly in producing sounds, and it is perhaps
m relation to this that the psychology of sex can first belaid to
come within the range of observation. Thus the field-cricket is
said to lower the tone of his song while caressing the female with
his antennae. In the parasitic forms dimorphism, as might be
expected, becomes very marked ; in Strepsiptera the males are free
and winged, while the females are blind and wingless, in fact
permanently larval Similar cases occur in other orders, the glow
worm being probably the most familiar instance. In parasitic or
abundantly nourished forms parthenogenesis very frequently
appears, the extreme case being presented by Cecidomyia, a fly
which exhibits rapid parthenogenetic reproduction in the larval
state. The dimorphism of many beetles, in which the male
fiequently acquires the most extraordinary specializations of
external form, has received especial attention from Darwin, whose
Dcsceni o/ Mm includes the fullest details. Here it is enough to
mention that Reichenau has recently pointed out the coexistence
ot the larger size and relative inactivity of the male with the
presence of these functionless outgrowths. The beautiful sexual
dimorphism so common among the Lepidoptera need not be more
ment.loned present; while the very remarkable sexual
differentiation of Hymenoptera (bees, ants, sawflies, &c.) may also
be assumed to be sufficiently familiar. See Insects, Ants Bees
In several orders {Diptera, Lepidoptera, Coleoptera) cases of dimor¬
phism occur among the females themselves, or even among the
males ; as many as three forms of females have been described in
certain butterflies.
The Molluscan series opens with the normally dioecious Lamelli-
branchs, of which some genera (most species of Ostrea, Pecten &c )
are, however hermaphrodite. The Pteropods, Pulmonates,’ and
Opisthobranchs are hermaphrodite ; the Prosobranchs, Heteropods
and Cephalopods unisexual. Though slight differences have been
described even in Lamelhbranch shells (Unio), and though the
internal anatomy of the essential and accessory organs is of very
high complexity, the extraordinary phenomena associated with
hectocotylization among the Cephalopod are the only marked
outward manifestations of that sexual dimorphism which reaches its
climax in. the Argonaut. (See Mollusca, Cuttle-fish.) The
umcates are usually hermaphrodite; Amphioxus, however is
unisexual (see Tunicata). ’ ’
Among Pishes hermaphroditism is extremely rare (Serranus).
ihe males are sometimes characterized by the modification of the
pelvic limbs as claspers &c., and are at the reproductive period
often readily distinguishable from the females by their brighter
colour or other cutaneous changes, such as ruffling of the ikin.
Male and female rays are readily also distinguishable by their teeth
and dermal defences. The hooked jaw of the male salmon gives
him a characteristic physiognomy during the breeding season. The
carp undergoes a sort of epidermic eruption at the same period ;
male and female eels, too, are said often to become distinguishable
MM11 C.0 0l“' a^d. shaPe- Stridulating apparatus may be present,
notably in the Siluroids. (See Ichthyology.) Among Amphi-
bians the bright dorsal crest of the male newt is perhaps the most
striking of sex distinctions, but many male frogs and toads have
vocal air sacs, epidermal callosities, and some (Cultripes, Pelobates)
possess a gland under the fore-limb. (See Amphibia. )
Among the Ophidians the males are smaller, and have longer and
more slender tails 5 the sexes, too, differ sometimes in colour and
markings. Male Chelonians, too, have sometimes longer tails and
claws and may even give voice. The submaxillary musk-gland
of the crocodile is especially active in the breeding season : the
lizards have remarkable throat-pouches and crests, which may be
chamele10 °r eVen COITesPon(l t° cranial outgrowths, as in the
But it is among Birds and Mammals that the observer of sexual
characters finds abundant and remarkable differences extending to
the minutest details, and showing how the higher evolution of
parental care which the inevitably prolonged embryonic life in-
voives and the wider range of sexual selection have co-operated in
modifying the whole organism. As might be expected, the lower
mammals show least of this ; but as we ascend the adult males
ecome differentiated from the females by the acquirement of
secondary sexual characters which are mainly either offensive and
, 1ci en®lv® aids for battle with each other, or which assist in gaining
tne admiration of the females ; and these may coexist or coincide in
very various degrees. Thus scent-glands are of common occurrence
10m the Insectivora (perhaps even from Ornithorhynchus) upwards,
.eater beauty of markings or more vivid colours are acquired,—
m many Anthropidae (baboons, &c.) the latter being of peculiarly
721
mostnSlv inntb local S^ths of hair often appear,
men! nf f i th, i1011 aud 111 many Anthropidae. The develon-
Sssffigmentiom ^ iS alS° t0° familiar to need thaPn
respecVthe tMs as in not a few other
hiXst stages of ^ t^n *lK! “^mals, have reached the
seem mfrpK 5eVola T ror here sexual characters no longer
individual life but^Vf ^ ®uPPleme.ntary to the apparatus^
ougiilv adai ted to 3 “Ration alike become thor-
functmns as it we™ r sex:dlfferences and the reproductive
u ;mv Trul rn n saturating the whole life, and producing so
and songd that H11-0118 in hal)its and character, in befuty
labours of 3 3 t0 be w°ndered at that the descriptive
those of the artisT fod 0rnitboloSist have constantly risen into
iZdo/Man ^ ^ the P°et- See Bieds> and twin’s
Nature and Determination of Sex.—It is not here pro¬
posed to enter upon the task of historical review and
criticism of the various theories of sex—which were esti¬
mated at so many as five hundred at the beginning of the
last century, or even to attempt any sketch of the present
very conflicting state of opinion on the subject.1
though our theories of sex may be still vague enough,
the greatest step to the solution has been made in the
general abandonment by scientific men of the doubtless
still popular explanation—in terms of a “natural tend¬
ency for the production of an excess of males or the like
It is now held that “ quality and quantity of food, elevation
of abode conditions of temperature, relative age of parents,
their mode of life, habits, rank, &c., are all factors which
have to be considered.” The idea that the problem of the
nature of sex is capable of being approached by empirical
observation of the numbers of different sexes produced
under known sets of conditions, and the obvious practical
corollary of this, viz., that the proportion of the sexes must
therefore be capable of being experimentally modified and
regulated, are conceptions which have steadily been acquir¬
ing prominence, especially of late. In short, if we can
find how sex is determined, we shall have gone far to
investigate sex itself.
One of the most crude attempts has been that of
Canestrim, who ascribes the determination of sex to the
number of sperms entering the ovum, but this view has
been already demolished by Fol and Pfliiger. The time of
fertilization has also and apparently with greater weight
been insisted upon ; thus Thury, followed by Diising, holds
that the sex of the offspring depends on the period of fer¬
tilization : an ovum fertilized soon after liberation produces
a female, while the fertilization of an older ovum produces a
male. This view has been carried a step farther by Hensen,
who suggests that the same should probably hold true of
the spermatozoa, and thus the fertilization of a young ovum
by a fresh sperm would have a double likelihood of result¬
ing in a female. There are some observations which
support this: thus Thury and other cattle-breeders have
claimed to determine the sex of cattle on this principle,
and Girou long ago alleged that female flowers, fertilized
as soon as they are able to receive pollen, produced a
distinct excess of female offspring.
Great weight has also been laid on the relative age of
the parents. Thus Hofacker, so long ago as 1828, and
Sadler a couple of years later, independently published a
body of statistics (each of about 2000 births) in favour of
the generalization (since known as Hofacker’s and Sadler’s
law) that when the male parent is the elder the offspring
are preponderatingly male : while, if the parents be of the
same age, or a fortiori if the male parent be younger,
1 As for reproduction in general, so for sex, the most convenient
starting-point is the work of Hensen (“Die Zeugung,” in Hermann s
Hdb. d. Physiologic), while other dissertations are to be found in the
leading manuals of zoology and botany, especially, however, in special
papers too numerous to mention. See also Reproduction, and for
fuller bibliographical details see Geddes, “On the Theory of Growth
Reproduction, Sex, and Heredity,” Proc. Roy. Soc. Edin., 1886.
XXL — 91