Taboo and Genetics | Page 4

distant from man, in structure and function, for profitable direct comparison.
Especially is this true with respect to sex, which they do not possess.
Parthenogenesis includes very diverse and anomalous cases. The term signifies the ability
of females to reproduce in such species for one or a number of generations without males.
Many forms of this class (or more strictly, these classes) have apparently become
specialized or degenerated, having once been more truly sexual. Parthenogenesis
(division and development of an egg without the agency of male sperm) has been brought
about artificially by Jacques Loeb in species as complicated as frogs.[1,2] All the frogs
produced were males, so that the race (of frogs) could not even be theoretically carried on
by that method.
The origin of sexual reproduction in animals must have been something as follows: The
first method of reproduction was by a simple division of the unicellular organism to form
two new individuals. At times, a fusion of two independent individuals occurred. This
was known as conjugation, and is seen among Paramecia and some other species to-day.

Its value is probably a reinvigoration of the vitality of the individual. Next there was
probably a tendency for the organism to break up into many parts which subsequently
united with each other. Gradually some of these uniting cells came to contain more food
material than the others. As a result of their increased size, they possessed less power of
motion than the others, and in time lost their cilia (or flagella) entirely and were brought
into contact with the smaller cells only by the motion of the latter. Finally, in colonial
forms, most of the cells in the colony ceased to have any share in reproduction, that
function being relegated to the activities of a few cells which broke away and united with
others similarly adrift. These cells functioning for reproduction continued to differentiate
more and more, until large ova and small, motile spermtozoa were definitely developed.
The clearest evidences as to the stages in the evolution of sexual reproduction is found in
the plant world among the green algæ.[3] In the lower orders of one-celled algæ,
reproduction takes place by simple cell division. In some families, this simple division
results in the production of several new individuals instead of only two from each parent
cell. A slightly different condition is found in those orders where the numerous cells thus
produced by simple division of the parent organism unite in pairs to produce new
individuals after a brief independent existence of their own. These free-swimming cells,
which apparently are formed only to reunite with each other, are called zoöspores, while
the organism which results from their fusion is known as a zygospore. The zygospore
thus formed slowly increases in size, until it in its turn develops a new generation of
zoöspores. In still other forms, in place of the zoöspores, more highly differentiated cells,
known as eggs and sperms, are developed, and these unite to produce the new individuals.
Both eggs and sperms are believed to have been derived from simpler ancestral types of
ciliated cells which were similar in structure and closely resembled zoöspores.[A]
[Footnote A: This evidence, which points to the conclusion that in the early origin of
sexual reproduction the males and females were differentiated and developed from a
uniform type of ancestral cell, quite controverts Ward's point that the male originated as a
kind of parasite.]
Having once originated, the sexual type of reproduction possessed a definite survival
value which assured its continuation. Sex makes possible a crossing of strains, which
evidently possesses some great advantage, since the few simple forms which have no
such division of reproductive functions have undergone no great development and all the
higher, more complicated animals are sexual. This crossing of strains may make possible
greater variety, it may help in crossing out or weakening variations which are too far
from the average, or both.
Schäfer[4] thinks that an exchange of nuclear substance probably gives a sort of chemical
rejuvenation and very likely stimulates division. At any rate, the groups in which the
reproductive process became thus partitioned between two kinds of individuals, male and
female, not only survived, but they underwent an amazing development compared with
those which remained sexless.
There came a time in the evolution of the groups possessing sexual reproduction, when
increasing specialization necessitated the division into reproductive and non-reproductive

cells. When a simple cell reproduces by dividing into two similar parts, each developing
into a new individual like the parent, this parent no longer exists as a cell, but the material
which composed it still exists in the new ones. The old cell did not "die"--no body was
left behind. Since this nuclear substance exists in the new cells, and since these
generations go on indefinitely, the cells