33. Sex, or male and female. In the slightly complex animals one individual produces both egg cells and sperm cells. But in the Siphonophora, or colonial jelly-fishes, studied in the last chapter, certain members of the colony produce only sperm cells, and certain other members of the colony produce only egg cells. If the Siphonophora be considered an individual organism and not a colony composed of many individuals, then, of course, it is like the others of the slightly complex animals in this respect. But as soon as we rise higher in the scale of animal life, as soon as we study the more complex animals, we find that the egg cells and sperm cells are almost always produced by different individuals. Those individuals which produce egg cells are called female, and those which produce sperm cells are called male. There are two sexes. Male and female are terms usually applied only to individuals, but it is evidently fair to call the egg cells the female reproductive cells, and the sperm cells the male reproductive cells. A single individual of the simpler kinds of animals produces both male and female cells. But such an individual can not be said to be either male or female; it is sexlessthat is, sex is something which appears only after a certain degree of structural and physiological differentiation is reached. It is true that even among many of the higher or complex animals certain species are not represented by male and female individuals, any individual of the species being able to produce both male and female cells. But this is the exception.

34. The object of sex.-Among almost all the complex animals it is necessary that there be a conjugation of male and female reproductive cells in order that a new individual may be produced. This necessity first appears, we remember, among very simple animals. This intermixing of body substance from two distinct individuals, and the development therefrom of the new individual, is a phenomenon which takes place through the whole scale of animal life.

The object of this intermixing is the production of variation. Nature demands that the offspring shall differ slightly from its parents. By having the beginnings of its body, the single cell from which the whole body develops, composed of parts of two different individuals, this difference, although slight and nearly imperceptible, is insured. Sex is a provision of Nature which insures variation.

35. Sex dimorphism.-As we have seen, almost every species of animal is represented by two kinds of individuals, males and females. In the case of many animals, espe

cially the simpler ones, these two kinds of individuals do not differ in appearance or in structure apart from the organs concerned with multiplication. But with many animals the sexes can be readily distinguished. The male and female individuals often show marked differences, especially in external structural characters. We can read

ily tell the peacock, with its splendidly ornamental tail feathers, from the unadorned peafowl, or the horned ram from the bleating ewe. There is here, plainly, a dimorphism-the existence of two kinds of individuals belonging to a single species. This dimorphism is due to sex, and the condition may be called sex dimorphism. Among some animals this sex dimorphism, or difference between the sexes, is carried to extraordinary extremes. This is especially true among polygamous animals, or those in which the males mate with many females, and are forced to fight for their possession. The male bird of paradise, with its gorgeous display of brilliantly colored and fantastically shaped feathers (Fig. 27), seems a wholly different kind of bird from the modest brown female. The male golden and silver pheasants, and allied species with their elaborate plumage, are very unlike the dull-colored females. The great, rough, warlike male fur seal, roaring like a lion, is three times as large as the dainty, soft-furred female, which bleats like a sheep.

Among some of the lower animals the differences between male and female are even greater. The males of the common cankerworm moth (Fig. 28) have four wings;

FIG. 28.-Cankerworm moth; the winged male and wingless female.

the females are wingless, and several other insect species show this same difference. Among certain species of white ants the females grow to be five or six inches long, while the males do not exceed half an inch in length. In the

case of some of the parasitic worms which live in the bodies of other animals, the male has an extraordinarily degraded, simple body, much smaller than that of the female and differing greatly from that of the female in structure. In some cases even-as, for example, the worm which causes 66 gapes in chickens - the male lives parasitically on the female, being attached to the body of the female for its whole lifetime, and drawing its nourishment from her blood (Fig. 29).

A condition known as parthenogenesis is found among certain of the complex animals. Although the species is represented by individuals of both sexes, the female can produce young from eggs which have not been fertilized. For example, the queen bee lays both fertilized and unfertilized eggs. From the fertilized eggs hatch the workers, which are rudimentary females, and other queens, which are fullydeveloped females; from the unfertilized eggs hatch only males-the drones. Many generations of plant lice are produced each year parthenogenetically that is, by unfertilized females. But there is at least one generation each year produced in the normal way from fertilized eggs.

FIG. 29. The parasitic worm (Syngamus trachealis), which

causes the " gapes" in fowls. The male is attached to the

female, and lives as a parasite on her.

Some of the complex animals are hermaphroditic-that is, a single individual produces both egg cells and sperm cells. The tapeworm and many allied worms show this condition. This is the normal condition for the simplest animals, as we have already learned, but it is an exceptional condition among the complex animals.

36. The number of young.-There is great variation in the number of young produced by different species of animals. Among the animals we know familiarly, as the mammals, which give birth to young alive, and the birds, which lay eggs, it is the general rule that but few young are produced at a time, and the young are born or eggs are laid only once or perhaps a few times in a year. The robin lays five or six eggs once or twice a year; a cow may produce a calf each year. Rabbits and pigeons are more prolific, each having several broods a year. But when we observe the multiplication of some of the animals whose habits are not so familiar to us, we find that the production of so few young is the exceptional and not the usual habit. A lobster lays ten thousand eggs at a time; a queen bee lays about five million eggs in her life of four or five years. A female white ant, which after it is full grown does nothing but lie in a cell and lay eggs, produces eighty thousand eggs a day steadily for several months. A large codfish was found on dissection to contain about eight million eggs.

If we search for some reason for this great difference in fertility among different animals, we may find a promising clew by attending to the duration of life of animals, and to the amount of care for the young exercised by the parents. We find it to be the general rule that animals which live many years, and which take care of their young, produce but few young; while animals which live but a short time, and which do not care for their young, are very prolific. The codfish produces its millions of eggs; thousands are eaten by sculpins and other predatory fishes before they are hatched, and other thousands of the defenseless young fish are eaten long before attaining maturity. Of the great number produced by the parent, a few only reach maturity and produce new young. But the eggs of the robin are hatched and protected, and the helpless fledglings are fed and cared for until able to cope with their natural