Page:Popular Science Monthly Volume 15.djvu/160

148 snows of thousands of winters, receiving new additions at its upper end, and at the same time melting away at its lower, is no bad representation of the long series of hereditary features, once variations, which form so large a part of every organism. If the glacier were not in motion, but stationary, so that the melting of the oldest portion and the additions to its upper end should gradually carry the body of ice up to higher and higher levels, we should have a very perfect parallel to the evolution of an organism by variation and heredity.

The steps in this progress are embodied in a long series of individuals, each of which is, either immediately or indirectly, the product of a fertilized egg or seed, through which the laws of heredity and variation act, to bind the separate individuals into a progressive whole. The seeds and eggs with which we are most familiar are highly complicated, and consist of the protoplasmic germ, which is intimately united to a mass of food destined to be converted into protoplasm during development.

The germ with its food forms the yolk of such an egg as that of the bird, and is surrounded by layers of albumen, which are also used as food, and by a complicated series of investing membranes. It originates in a special organ, the ovary, and is incapable of perfect development until it has been fertilized by the male reproductive element. In its earliest stage of growth it is simply one of the cells or histological elements of the ovary, but as it grows it soon becomes very much larger than an ordinary cell, and its protoplasm becomes filled with food material, and the outer layers and walls are added to it. In many animals, the external envelopes are wanting, and the egg is simply a very large ovarian cell, filled with food material, and capable of developing, under the influence of the male element, into a new organism. In still other animals the food-yolk is wanting, and the egg is small, and does not differ from an ovarian cell; and in still other animals the ovaries are lacking, and cells may become specialized as ova in various parts of the body.

The series is so complete that we may be certain that we are comparing strictly homologous structures, and we may therefore conclude that the egg is nothing but one of the cells of the body, which may, when acted upon by the male element, develop into a new organism, substantially like its parents, with some of the individual peculiarities of each of them, and also with new peculiarities of its own.

From the necessity for impregnation in most cases, it has been assumed that the essential function of the male element is to quicken the germ, and thus start the process of development. It is true that it does have this function in many cases; but comparative study shows that the egg itself is alive, and does not need quickening, and that this must be regarded as a secondary and derived function of the male element, not the essential and primitive function.

That this is the case is shown by the fact that, while the earlier