To the scientist, then, the wonder comes, not in the fact that the child resembles the parent, but in the fact that the child is not identical with the parent. Part of the explanation of this lack of identity, or, as it is known to science, variation, is due to the fact of sexual reproduction; that is, to the fact that the child has two parents instead of one. [Sidenote: Microscopic study of reproductive cells] The physiological process which takes place in the union of two reproductive cells is truly most wonderful. Of late years this has been studied under powerful microscopes and has resulted in some very wonderful revelations of the mysteries of Nature. [Sidenote: Chromosoms in different species] The nucleus (center of growth) of the parent cells contains little thread-like structures known as chromosoms. These chromosoms are considered to be the physical basis of heredity. In each species of animal there is a definite and a different number. [Sidenote: Action of the chromosom] When the sperm-cell unites with the female or germ-cell, these thread-like chromosoms pair off and unite each chromosom with the corresponding structure from the other cell. The combined structures then divide, and half of each chromosom is cast out of the cell-nucleus, and plays no part in the life of the future being; the other half is retained and divides as each new cell is formed. Thus we see that every part of the new individual is the result of the fusion or combination of the two parents. This explains the variation of inheritance, and through this source must be traced all traits of heredity. After the original fusion of these microscopic physical elements of heredity, the future development of the individual is wholly a matter of environment and nutrition. WHAT HEREDITY IS [Sidenote: General characteristics due to heredity] What heredity is and what it is not will now be considered in a practical way. It is clearly a matter of heredity that a man is born a man and not a monkey. Likewise, it is clearly a matter of heredity that distinguishes the various races of men. We could go farther and trace out and describe many of the physical distinctions which mark families, and even individuals, such as general size of frame, form of countenance, color of hair and eyes, etc. [Sidenote: Characteristics not attributed to heredity]