uals, then, of course, it is like the others of the slightly complex animals in this respect. But as soon as we rise higher in

FIG. 125.-Fertilization of Petromyzon fluviatilis: A, Sperm nucleus in periphery of the egg plasm; B, sperm nucleus in periphery of the egg plasm, and egg nucleus approaching; C-E, fusing of the egg and sperm nuclei, and appearance of the asters; F, cleavage of nucleus. (After Herfort.)

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 cannot be said to be either male or female, it is sexless

that 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.

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 seems to be the production of variation; at least it would seem that variation must result from such a mode of generation. By having the beginnings of an organism's body, the single cell from which this whole body develops, composed of parts of two different individuals, a difference between the offspring and the par

ents, although it may be slight and imperceptible, is insured. Sex is a condition of nature which is one, at least, of the causes of variation.

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, especially the simpler ones, these two

kinds of individuals may 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 readily 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 polyga

mous 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. 128), 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. 129) have four wings; 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 it in structure. In some cases even-as, for example, the worm which causes "gapes" in chickens-the male lives parasitically on the female, being attached to her body for its whole lifetime, and drawing its nourishment from her blood (Fig. 130).

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.

However the beginnings of the new organisms are produced, whether asexually or bisexually (whether, that is, by simple division, budding, sporulation, or as true but unfertilized eggs, or as eggs with a nucleus made by the fusion of two germinal nuclei from male and female individuals respectively, or from an hermaphroditic individual), this new organism in embryo has a shorter or longer course of development and growth to undergo, before it, in turn, is in condition to produce new individuals of its kind.

FIG. 130. 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.

Certain phenomena are familiar to us as recurring inevitably in the life of every animal which we familiarly know. Each individual is born in an immature or young condition; it grows (that is, it increases in size) and develops (that is, changes more or less in structure) and dies. These phenomena occur in the succession of birth, growth, and development, and death. But before any animal appears to us as an independent individual that is, outside the body of the mother and outside of an egg (i. e., before birth or hatching, as we are accustomed to call such appearance)-it has already undergone a longer or shorter period of life. It has been a new living organism hours or days or months, perhaps, before its appearance to us. This period of life has been passed inside an egg, or as an egg,