HEREDITY
How often we hear someone remark upon the wonders of heredity. People are astonished because John should look like John's father. As a matter of fact, the astonishment should come the other way. The child is but a continuation of the life of the parents. The cells from which the child develops have within them the power to grow and to produce individuals like the parents. This is wonderful, but it is only another form of the wonder of a willow twig growing into a willow tree when placed in moist earth.
Why the child is not identical with parents
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.
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.
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.
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.