An individual which produces both sperm-and egg-cells within its body is termed an hermaphrodite. Very few hermaphrodites exist among the vertebrates, although they may be found in one or two species (e.g., the hagfish). There are no truly hermaphroditic mammals, i.e., individuals in which both the male and the female germ cells function, except perhaps in rare instances.

Sexless or asexual reproduction assumes various forms. What is usually considered the most primitive of these is fission or simple division, in which the cell divides into two equal, identical parts. There is of course no suggestion of sex here. It is fairly safe to assume that life began thus in the world, as neuter or sexless—i.e., with no suggestion of either maleness or femaleness.[[A]]

[A]

This asexual type of reproduction has been misinterpreted by a whole school of non-biological writers, who have followed the lead of Lester F. Ward, in his classification of these neuter-organisms as females. Ward says ("Pure Sociology," Ch. 14): "It does no violence to language or science to say that life begins with the female organism and is carried on a long distance by means of females alone. In all the different forms of asexual reproduction from fission to parthenogenesis, the female may in this sense be said to exist alone and perform all the functions of life including reproduction. In a word, life begins as female" (p. 313). Adding to this statement the assertion that the male developed at first as a mere parasite, in the actual, physical sense, Ward proceeds to build up his famous Gynæcocentric Theory, which is familiar to all students of social science, and need not be elaborated here. It is obvious that a thorough biological knowledge destroys the fundamental concept on which this theory is founded, for there is no doubt that life begins as neuter or sexless, and not as female.

There are a number of other forms of asexual reproduction, or the "vegetative type" (Abbott's term, which includes fission, budding, polysporogonia and simple spore formation). Budding (as in yeast) and spore formation are familiar to us in plants. Such forms are too 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]]