Taboo and Genetics | Page 4

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 alg?.[3] In the lower orders of one-celled alg?, 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 zo?spores, 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 zo?spores. In still other forms, in place of the zo?spores, 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 zo?spores.[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.
Sch?fer[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 are in a sense "immortal" or deathless. In a one-celled individual, there is no distinction between germinal and bodily functions. In the more complicated organisms, however, there are innumerable kinds of cells, a few (the germ cells) specialized for reproduction, the others forming the body which eats, moves, sees, feels, and in the case of man, thinks. But the germ-cells or germplasm continue to be immortal or deathless in the same sense as in the simplest organisms. The body, in a historical sense, grew up