FIG. 101.-Diagram showing arrangement of bones in the hand or foot of various animals: 1, man; 2, gorilla; 3, orang; 4, dog; 5, sea lion; 6, dolphin; 7, bat; 8, mole; 9, Ornithorhynchus. (After Haeckel.)

If this is true, it would appear that nervous overstrain of the parent is unfavorable to normal nerve development of the offspring. This would be apparently a case of transmission of parental conditions, as above indicated, and not one of true heredity.

It may be conceived that, at the moment of impregnation, the resultant germ cell is sexless. It begins its development at once, and, in the higher animals, turns very soon toward the formation of those structures which distinguish the one sex or the other. Each individual ultimately becomes either male or female. Relatively few animals, and those among the lower

FIG. 102.-Limb skeletons of extinct and living animals, showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4, Aëtosaurus; 5, Plesiosaurus; 6, Ichthyosaurus; 7, Mososaurus; 8, duck.

forms, are ever really hermaphrodite, or representative of both sexes at once.

Among the invertebrate animals the numerical relations of the sexes are subject to great variation. Among vertebrates, in general, the sexes are practically equal in number, as is shown by count of large series of individuals. This is true whether the species be monogamous, polygamous, or promiscuous in its sex relations. It is therefore apparent that the sex tendencies in

the germ are held on a very fine balance. A very slight impulse the one way or the other determines the sex direction the embryo shall take. Although much investigation and very much. speculation have been devoted to this problem, it is still unsolved. We are not able, in the vertebrate animals, nor in fact in animals generally, to determine the nature of the stimulus, or of any of the various impulses, if more than one exists, which

FIG. 103.-Limb skeletons of various animals, showing homologous bones: 9, Ornithorhynchus; 10, kangaroo; 11, Megatherium; 12, armadillo; 13, mole; 14, sea lion; 15, gorilla; 16, man.

leads the individual germ cell to develop as male or female. It is also possible that each germ cell is really bisexual from the beginning. One sex or the other becomes dominant and the other recessive as the embryo develops. But in this event we are still in doubt as to the nature of the determining factor or

The latest studies of the problem are chiefly concerned with an attempt to determine whether or not there exists a chromosome sex determinant, and whether sex determination may not be brought under Mendel's law of heredity (see later paragraphs in this chapter) in a modified form.

stimulus. Among ants and the social bees and wasps the males develop parthenogenetically from unfertilized cells, the fertilized cells yielding either females or workers which are sterile females. But this specialized mode of development is peculiar to particular groups. For a few lower species it has been ascertained that variation in nutrition may be a factor in sex determination. Favorable nutrition seems to increase the number of females. Most higher plants are hermaphrodite, the central leaves (carpels) in the bud which becomes the flower, yielding ovules or fe

FIG. 104.-Limb skeletons of various animals, showing homologies of the bones; at left, mole; next, giraffe; next, bat; next, porpoise.

male germ cells. The next whorl (stamens) yields male germ cells or pollen. The outer whorls (corolla, calyx) serve as protective organs only, and are without sex.

The bonds of union among organisms which stand at the basis of all classification are known as "homologies" (Figs. 101104). A homology is a real likeness, as distinguished from one, merely superficial or apparent. To superficial likeness we give the name of analogy. Homology means fundamental identity of structure, as distinguished from incidental similarity of form or function. Thus, the arm of a man is homologous with the foreleg of a dog, because in either we can trace deep-seated resemblance or homologies with the other. In each detail of each bone, muscle, vein, or nerve of the one we can trace the

corresponding details of the other. But in comparing the arm of man with the "limb" of a tree, the arm of a starfish, or the foreleg of a grasshopper, we find no correspondence in details. In a natural classification, or one founded on fact, organisms showing the closest homologies are placed together. An artificial classification is one based on analogies. Such a classification might place together a cricket, a frog, and a kangaroo, because they all jump, or a bird, a bat, and a butterfly, because they all fly, even though the wings are very differently made (Fig. 105) in each case.

The very existence of such terms as animals and plants, insects and mollusks imply relationships, and relationships in different degrees. Classification is the process of reducing our knowledge of these grades of likeness and unlikeness to a system. By bringing together those which are fundamentally alike, and separating those which are unlike, we find that

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FIG. 105.-Diagram of wings, showing homology and analogy: a, wing of fly; b, wing of bird; c, wing of bat.

these traits are the outcome of long-continued influences. Classification is defined as "the rational lawful disposition of observed facts." It rests on the results of the operations of natural laws, or forces which bring about inevitable results.

For it is a matter of common observation that the closest

homologies are shown by those animals which have sprung from a common stock. The fact of blood relationship shows itself always in homology. So far as we know, homology is never produced in any other way, therefore the actual presence of homologies among animals or plants implies, as we shall see in a later chapter, their common descent from stock possessing these same characters. In our primitive use of the trunk of the tree to imply unity in life, we can see that this trunk represents