The instinct of propagation is normally derived from the impulse for the satisfaction of protoplasmatic hunger. In lower animal life the incipient sexual union is effected by two exhausted cells coming together for the mutual exchange of nuclear material. This mode represents the first step in the scale of conjugation.
In the lowest form of unicellular life, as the schizomycetes (yeast) or bacteria, the necessity for conjugation does not appear to exist. In the reproduction of these unicellular organisms the animal simply divides into two, the division of the nucleus, as a rule, preceding that of the cytoplasm by a more or less karyokynetic method.
The ordinary protozoon does not form a composite structure. It divides and multiplies, but the products of the division do not remain together, they leave each other and lead a separate existence. Hence there is no real death in these animals. In the metazoa, or many-celled animals, only the reproductive cells may escape death and continue to live in the offspring. The somatic cells, or the body, die after a longer or shorter period. The unicellular animals do not possess any somatic cells, they are all reproductive cells. Hence we may rightly speak of the “immortality” of the protozoa.
The method of binary fission or splitting, by which the body of the parent becomes divided into two equal parts, into halves, is the simplest method of multiplication. This method is made use of in the amoeba. In this kind of reproduction there is no parent nor child. The children, the new amoebae, are simply the parent cut in two (vide Page 60).
The next simple generation is budding, which is the breaking off of a part smaller than half from a certain individual. The budded-off part has the capacity of growing into a new individual like the parent. This mode of reproduction is found in the hydra.
Another simple mode of generation is that of sporulation. Here the interior of the body of the individual subdivides into more than two parts. Sometimes the parts number many hundreds and are called spores. These three methods are the simplest modes of generation and are exclusively found in the lowest forms of unicellular organisms.
When we go a step farther, in the class of unicellular protozoa, the simple mode of multiplication continues, in most forms, only for a certain number of generations. Then the necessity for conjugation, i. e., for a temporary or permanent fusion with another individual, sets in. If this conjugation be prevented, the animal soon degenerates and dies.W
The simplest terms of conjugation are found in Chilodon, a minute fresh-water infusorium, which multiplies for a considerable length of time by transverse division. After a time, however, the physiological necessity for conjugation sets in. The different animals place themselves side by side, in pairs, and partly fuse together. The nucleus of each individual divides now into two portions, one of which passes from each infusorium into the other to unite with the half of the nucleus that remained stationary. The two animals then separate, each having received a half of the nucleus of the other. Thereupon a period of renewed activity for each ensues, manifested by rapid growth and multiplication by division, until a certain weakening in the vital activities indicates the periodically recurring necessity for conjugation.
The next step in conjugation is found in those animals in which after their fusion the two animals do not separate any more, as found in the fission of monads, which is preceded by the absorption of one form by another. One monad is fixed upon the sarcode of another, and the substance of the lesser, which is the lower one, passes into the upper one. In about two hours the merest trace of the lower one is only left, and in four hours fission and multiplication of the larger one has taken place.
From these two modes of generation it is easily seen that the impelling force leading to conjugation is, as Rolph puts it, simply cell hunger. These modes of multiplication are also found in the simpler forms of colonial protozoa,X where the cells are not yet differentiated and all the members of the colony take part in reproduction.
In the next higher class of colonial protozoa, the first differentiation of the members or cells of the colony takes place. One part of the cells is set apart to continue the task of reproduction, while the main body of the colony does not participate any longer in the function of generation. As a rule, the reproductive cells are divided into two kinds, and these two kinds of cells conjugate with each other. The conjugation takes place either between different members of the same colony or between members of different colonies of the same species.
The conjugating individuals are similar in the simpler kinds of colonies. In the higher forms the two conjugating cells are easily seen to be very different from each other. One kind of cells is large, spherical and inactive. They are for this reason called egg-cells. The other kind of cells is small, with ovoid head and tapering tail and free-swimming. For this reason they are called sperm cells.
When the differentiation between the reproductive cells into two kinds has taken place, they are called sex cells. The sex cells are usually situated in groups, and the groups are called sexual glands. The groups of sperm cells are called testicles and those of egg cells are called ovaries.
In the lower classes in the animal kingdom the male and female glands are situated at different parts of the same animal, or near each other. Sometimes the same gland is producing both kinds of cells, male and female. In the hermaphroditic species of round worms, for instance, it is found that, when the reproductive organ is fully formed, it functions first as a testicle. The germ-cells at the anterior end of the sexual gland begin to divide rapidly and become small spermatozoa which are stored up in a receptacle of the uterus. Later on other cells, also situated at the anterior end of the sex-gland, begin to grow larger, store up yolk and become large egg-cells. They then enter the uterus and become fertilized by their own spermatozoa. This mode of fertilization is the so-called self-fertilization. It is found in the slightly complex animals, as the tape-worm or the leech, where one and the same individual produces both egg-cells and sperm-cells. In colonial jelly-fishes certain members of the colony produce only sperm cells, and certain other members produce only egg-cells.
In the hermaphroditic animals the sperm cells of the same individual may fertilize their own egg-cells or ova, as in the feat-worm (self-fertilization), while in the earth worm, although it is also hermaphroditic, cross-fertilization takes place. Two earth-worms mutually fertilize each other, the sperm cells of one fertilize the ova of the other, and the sperm cells of the latter fertilize the ova of the former.
Fertilization in plants.—This mode of cross-fertilization is the one generally found in phanerogamous (φανερός = apparent and γάμος = marriage) plants.
In the lower forms of the organic world the line of demarkation between plant and animal is somewhat blotted, and the identity of the mode of fertilization does not need to be particularly emphasized. In the higher forms the division is generally made by the mode of the assimilation of food and of motion. Plants generally break up the inorganic compounds into separate elements and recombine them into organic compounds, or potential energy. These organic compounds serve then the animal as food or as source of energy. The inorganic compounds being, as a rule, ubiquitous, the plants have hence a certainty of food supply and do not depend upon the facility of motion. They are, therefore, marked by a fixity to the soil. They are also surrounded by an outer skeleton, or a coat of cellulose and are less affected by outer stimuli; hence they possess slight consciousness. For the animals the food-supply is more uncertain, hence they need greater movement. For this reason they generally lack the external skeleton and hence possess higher consciousness.
Apart from this division plant and animal are closely related cousins, and the mode of fertilization of most of the higher plants is about the same as in the hermaphroditic animals.
The sexual organs of the phanerogamous plants are represented by the flowers.Y Showy flowers consist usually of four sets of organs, pistils, stamens, petals, and sepals. The sepals taken together constitute the calyx, the petals taken together constitute the corolla.
The essential organs of the flower are the stamens and pistils. The stamen represents the male organ, and the pistil the female organ of the plant, while the calyx and corolla form the floral envelopes, or the so-called perianth and are analogous to the external genital organs in the animal. Flowers which contain all the four sets of organs are said to be complete flowers, those which have the essential organs only are called perfect flowers.
Complete and perfect flowers are hermaphroditic. The male organ or the androcele and the female organ or the gynocele are situated in one and the same flower. The unisexual flowers are those where the stamens and the pistils are produced on separate flowers, as in the willow. Stamen and pistil may not only be produced on separate flowers, but the staminate and pistillate flowers themselves may be borne on different plants, as in hickory, hazel, or Indian corn. Such plants are called dioecious or of two households. When both kinds of flowers appear on the same individual, the plant is called monoecious or of one household.
The stamen, or the male sexual organ of the flower, consists of a hollow portion called the anther, which is borne on a stalk called the filament. Inside of the anther is found a powdery or pasty substance called the pollen. The shape of the anther and the way in which it opens depend largely upon the way in which the pollen is to be discharged and how it is carried from flower to flower. As a rule, the anther opens by the cells being split length-wise, or by little holes at the top.
The pollen in many plants is a fine dry powder, in others it is somewhat sticky or pasty. The forms of the pollen grains are various. Each pollen-grain consists of a single cell and is covered by a thick outer and a thin inner wall or coat. At the outer coat there are spots at which the inner coat of the grain is finally to burst through the outer one, pushing its way out in the form of a slender thin-walled tube. The contents of the pollen is a thickish protoplasma, full of little opake particles, and usually containing grains of starch and little drops of oil.
The pistil, or the female sexual organ of the flower, usually consists of a small hollow chamber called the ovary, which contains the ovules, and of a slender portion or stalk, called the style. At the top of the latter is found a ridge, knob, or point which is called the stigma.
The stigma consists of cells loosely arranged over the surface. These cells secrete a moist substance, to which the pollen grains adhere when they come in contact with the stigma. Beneath these superficial cells, running down through the style, there are found long cells, with intermediate spaces, through which the pollen tube reaches the ovary.
The ovules are not borne indiscriminately by any part of the lining of the ovary. They grow in a line running along one side of the ovary, as may be seen in the pea pod. This ovule-bearing line is called the placenta.
The ovule usually exists as a roundish or egg-shaped mass, with a small opening leading into the apex. This opening leads to a sac inside the ovule, which is filled with a soft protoplasmatic material and cells and is known as the embryo sac. Minute cells are found at the apex of the ovule from the development of which the embryo is produced, after the union of the pollen with a cell at the apex of the embryo sac of the ovule has taken place.
Fertilization.—As soon as the pollen grain lodges on the stigma, it begins to form into a pollen tube. In more or less time it makes its way through the style into the ovary. It then penetrates the opening at the apex of the ovule, reaches one of the cells and transfers its nucleus into an egg-cell. The latter begins at once to form cell-walls and increases by continued subdivision to the plant embryo. Only one pollen tube is necessary to fertilize each ovule. Still plants produce more pollen than ovules—the ratio is from 1:8 to 1:1000—because so many pollen are lost on their way to the ovules.
The mechanism of the forwarding of the pollen to the ovule varies in different plants. In self-fertilization or in those plants in which the ovule may be impregnated by the pollen of the same flower, fertilization is comparatively easy. But in a great many plants cross-fertilization is the rule. In order to accomplish the most successful fertilization, the pollen must come from another plant of the same species. Here nature has devised different ways to carry the pollen from one plant to another.
In the first place, there is the wind which accomplishes the task of forwarding the pollen. The wind-fertilized flowers have dry and powdery pollen, and the pistils are feathery, adapted to catch flying pollen grains. The flowers are characterized by their inconspicuousness. They are usually greenish, without any odor or nectar.
Another device to forward the pollen is by way of insects. Most of the showy, sweet-scented or otherwise conspicuous kinds of flowers are entirely dependent for fertilization on the transference of pollen from one plant to another by insects. The showy colors and odors serve to attract insects to visit them for their nectar. Insects and flowers are interdependent upon one another. For many insects depend mainly upon the nectar and pollen of flowers for their food. These insects usually visit only one kind of flower during the day and thus carry only one kind of pollen. Butterflies, moths, and most of the bees go straight from one flower to another and carry a good deal of pollen entangled in the scales or hairs of their bodies. On its way, the insect leaves a good deal of the pollen on the stigma of the pistil and becomes dusted with new pollen to be carried to other flowers.
The means to attract insects are threefold: nectar, odor and color. The nectar is a sweet liquid which the flower secretes by means of nectar glands. The latter are usually situated near the base of the flower. Other plants attract insect visitors by giving up a sweet scent. These are especially the small flowers, like the mignonette or evening primrose. The color is another means by which the flower attracts insects and birds. The color of the flower is, as a rule, due to showy petals. Different kinds of insects are especially attracted by different colors. Some flowers with very long tubulated corollas depend entirely upon birds with long beaks to carry their pollen for them.
In complete and perfect flowers, where stamen and pistil are present in the same flower, self-fertilization would be the rule, if there were not certain means for its prevention. In the first place the pollen of another plant frequently prevails over that which the flower may shed over its own pistil. When both kinds are placed over the stigma, at the same time, it is the foreign pollen which causes fertilization. Another means to prevent self-fertilization consists in the stamens and pistils maturing at different times. The insect visitor, on its way to the nectary, brushes against the ripe stamens of a certain flower in its earlier stage. It cannot deposit the acquired pollen upon the stigma of the same flower and thus cause self-fertilization because the pistil is not ripe yet. But, in flying to a flower in the later stage, when the stigma has already ripened and the stamens have shed all their pollen, the insect will lodge the pollen first acquired on the ripe stigma, and in this way produce the desired cross-fertilization.
Sex-differentiation.—When we rise higher in the scale of animal life we find that the egg-cells and sperm-cells are almost always produced by different individuals. Those which produce egg-cells or ova are called female, and those which produce sperm-cells or spermatozoa are called male animals. The formation of a new being results in these species from the conjugation of two cellular elements of two different animals. In this way the origin of the organism, or the zygote from which the new individual develops is composed of parts of two different individuals, and a difference between the offspring and parents is insured. The intermixing of body-substance from two distinct individuals and the development therefrom of the new individual produce variation.Z
When the scale of animal life is reached where egg-cell and sperm-cell have their habitats in different individuals, the attraction between ovum and spermatozoön, which is based upon a kind of erotic chemotropismus, is transferred to the two hosts who harbor the two different sex-cells. Thus the erotic chemotropismus between two cells has now grown to sex-attraction between two animals. But even at this stage the attraction sometimes exists only between the sex-cells, namely when fertilization takes place outside of the mother’s body. The best example of outside fertilization are the fishes. The female fish contains the roe, which is a mass of small eggs. At the proper time the female lets the roe fall on the ground of rivers or the dark bottom of lakes, etc., at a favorable place called spawning bed, secure against enemies. The male fishes swim over the spawn and pour their semen or milt over it. The two kinds of cells attract each other like iron and magnet. When the milt has reached the spawn, the eggs are fertilized and develop into new fishes.
In the next higher class, in birds, fertilization is internal, and only the development is external. The female bird has an ovary containing a large number of eggs or ova, within its body. The male animal possesses an organ serving for the introduction of the semen into the body of the female. The fertilized egg is laid and develops outside, either by the sun-heat or hatched by the mother’s warmth.
The last step in generation is reached in the mammals. Here not only fertilization is accomplished within the body of the female, but also the entire development of the fertilized ovum is carried out in the uterus, inside the female body. Hence mammals give birth to living mature young.
In these higher animals the attraction of the sex cells has been definitely transferred to, or rather changed into, the attraction of the hosts of these cells or into the sexual instinct. The sex-instinct is, therefore, if not identical with the instinct of propagation, thus an offshoot of the latter instinct.
In the more advanced species of the higher animals the instinct relating to the preservation of the kind is composed of three definite impulses. There is first the impulse relating to the act of conjugation, universal in all organic life. The average individual grows up surrounded by others of the opposite sex and normally unexcited by this difference until the age of puberty approaches. Then some day, the boy or girl find arising within an incipient impulse which, naturally, were there no artificial restrictions, would lead them to exercise their sexual powers, just as found among animals.
The second impulse relates to the pursuit and attraction of mates. It is first found among the lower types of the animal kingdom. The sweet odors and showy colors found in flowers are nothing else but the means of attracting the insects that carry the pollen for fertilization. The bright colors of some animals, especially among birds, and other animal adornments also serve to attract the mates. The woman’s love for finery also emanates from no other source than from the impulse of pursuit and attraction of mates.
The third impulse stands in relation to permanent mating and the protection of the young. It is the impulse relating to the germs of family life, and is present among those animals that have gained a higher place in the ascending scale of complexity and whose young are more difficultly provided for.AA
Through the whole range of animal life, where the period of infancy is comparatively short (e. g. horse, dog, cat, etc.), the male part is ended with impregnation or copulation. The nursing of the young is left to the female,AB and she will sacrifice her own life in the protection of her breed. In animals whose young are not easily provided for (e. g. fox, wildcat, eagle, sparrow, pigeon, stork, etc.), not only the mother will sacrifice her own existence in the protection of her offspring, but also the father will do his utmost in the interest of his young ones. In these animals the males remain attached to the females they have secured at the first period of oestrum even after the time of propagation has passed, provide mutually for their offspring until the latter can provide for themselves, and at each succeeding period of rut, yield again to love and never seek a new mate until the old one dies. Ernest Thompson Seton found that hawks practise monogamy and that wolves consort for life, and, in case of death, the survivor remains alone. The Canadian wild goose, when it has lost its mate, will never seek another. Thus the instinct of permanent mating or of monogamic marriage, is a phenomenon already found among many animals.
Now, among all animals, the prenatal period and the period of the maternal feeding are almost the longest in man. The helplessness of the human infant is unique among the creatures of the animal kingdom. The new-born baby is devoid of nearly all instinctive capacities, except the taking-in and assimilation of food. It is unable to stand or wander in search of food. It is nearly blind and deaf. It is perfectly naked, without fur or feathers, and hence is in need of a certain amount of heat, being injured by the least draught. It is in need of the utmost cleanliness, still it is unable to keep itself clean. It is unable to fast longer than a few hours. In short, the human infant is the most complete picture of helpless dependence. Hence without the help and strength of fatherhood, afforded to motherhood, the human race could not have survived the primitive stage, when couples still lived separated. The length and feebleness of human infancy required a union of male and female of considerable duration. By the time the last child was able to emancipate itself from the parental protection, the period of sexual activity had been passed. Permanent mating among men, especially in the prehuman stage, was a condition sine qua non, all the preachings of the free-lovers to the contrary notwithstanding.AC
Permanent mating is, therefore, a natural impulse among human beings. It is necessary for the protection and preservation of the lives of a lesser number of offspring of a higher grade. Permanent mating is, hence, of fundamental racial value. In man this impulse is of a more complex form and broader range, which leads the individual to wish not for momentary excitement, but for a permanent union, for a home, for a family. Even to the man who regularly indulges in meretricious venery there comes a time when these bonds of mere passion do not satisfy any longer. He begins to crave for a permanent mate and a home. This impulse appears later in life and is not present in the earlier impulse for mere conjugation, arising at the period of puberty. The amatory feelings at that time of life are bestowed upon the first pleasing individual of the opposite sex and are seldom of long duration. They are transferred from individual to individual. Later in life, after character is formed, there comes for men and women the dawn of a deeper affection, which involves bonds of stronger form and more permanent type than any that mere passion can arouse. This unconscious and involuntary craving for a permanent mate and home is of an altruistic nature. It has been wisely implanted in the interest of the race. There is nothing egotistical connected with this impulse. Personal gratification does not enter into consideration, for the joy and pleasure connected with personal satisfaction may be found with any temporary mate.