Reproduction

396S

Republic

dally modified cells, known respectively as
ova and spermatozoa, which after • sexual
union has occurred become capable of de-
veloping into new organisms. This is the
ordinary process of sexual reproduction, as
known alike in the higher plants and the
higher animals. Another type of reproduction
consists in the liberation of clusters of the
ordinary body cells, which are capable of

Reproduction by Budding.
A, Ovary, with ovum; B, fully
formed bud, with mouth and
tentacles.

growing into a new organism. In plants,
vegetative reproduction is very frequent—
the tubers of the potato, the subsidiary bulbs
produced by most bulbous plants, the run-
ners of the strawberry, and so forth.

Sexual reproduction by the union of male
and female cells is practically universal in
multicellular plants and animals. In some
cases, however, female cells are produced
which without sexual union are capable of
development. Such ova are described as par-
tkeno genetic (See PARTHENOGENESIS) , and oc-
cur, for example, in some insects. All meth-
ods of reproduction are costly to the indi-
vidual ; wherefore reproduction does not nor-
mally occur until growth has almost ceased—
that is, until the individual has reached its
maximum development. It is also a common-
place of biology that the rate of reproduction
is highest in unspedalized organisms, and
tends to diminish with progressive specializa-
tion. The reason for the progressive diminu-
tion in higher forms in the number of off-
spring produced is not far to seek. It is
better for the species that relatively few
should be produced, with increased chance of
surviving to maturity, than an enormous

number where the chances of an individual
surviving are small, because the agents of
elimination which act in the earlier stages are
largely non-selective in their mode of action
If it is possible to protect the young through
their early stages, as is done by the parents
in more specialized forms, then, by leaving
the more fit to breed, the standard of the
species is always being raised; hence the bio-
logical justification of the parental care
shown by birds and mammals. See BIOLOGY;
EMBRYOLOGY; HEREDITY; CELL.

Reptiles (Reptilia), a very large class of
vertebrate animals, including Tortoises and
Turtles, Lizards of many kinds, the divergent
New Zealand 'lizard' Sphenodon, Snakes, and
Crocodilians—five distinct orders with living
representatives, but including also many or-
ders of wholly extinct types, such as Ichthyo-
saurs, Plesiosaurs, and Dinosaurs.

Reptiles occupy a central position in the
Vertebrate series: beneath them are Amphi-
bians and Fishes, above them are Birds and
Mammals. They begin the series of higher
Vertebrates. Reptiles are cold blooded, the
temperature of the body not greatly exceed-
ing that of the surrounding medium; the
heart is three-chambered, except in Croco-
dilians, where four chambers first occur; the
body is covered with scales, with which sub-
jacent bony plates or scutes are sometimes
associated; the great majority are oviparous,
while in some the eggs are hatched within
the mother. A general classification of liv-
ing reptiles is as follows: (i) Rhynckoce-
phalia, including only Sphenodon; (2) Lacer-
tilia, or lizards; (3) Ophidia, or snakes; (4)
Chelonia, or tortoises and turtles; (5) Croco-
dilza, or crocodiles and alligators. See TOR-
TOISES AND TURTLES; LIZARDS; SNAKES;
CROCODILES; ALLIGATOR.

Republic, a form of government in which
the sovereign power is vested, not in a hered-
itary ruler or in a ruler elected for life, but
in the body of citizens, or in a more or less
privileged section of them. According to the
constitution of the governing body, republics
have varied from the most exclusive oli-
garchy to a pure democracy. The several
republics of Greece and that of Rome were,
at the outset at least, aristocratic communi-
ties. The mediaeval republics of Venice,
Genoa, and the other Italian towns were also
more or less aristocratic. The most important
of modern republics is that of the United
States of America, where pure democracy has
been tried on a scale unknown elsewhere. Ex-
cept during the short-lived empire of 1863-7,