Page:Popular Science Monthly Volume 16.djvu/183

 highest organisms each part or organ has but one function to perform, and therefore does it thoroughly. You will observe that the final cause, the end to be attained, the raison d'être, in all this process is better work, a better result. Now, my object will be to bring the origin of sex under this general law—to show some of the steps, and that each step was attended with better results.

—You already know that there are two fundamentally distinct kinds of reproduction, viz., sexual and non-sexual—so distinct, indeed, that there seems to be no possible connection between them. But remember that not only are our distinctions in science far more trenchant than they are in nature, but also that the distinctions in nature now are far more trenchant than they were in early geological times. It is the peculiarity of modern science, under the guidance of the doctrine of evolution, that it loves to dwell upon the gradations rather than upon the distinctions—it seeks for the missing links which make the chain of nature continuous. Now, there are several grades of sexual as well as of nonsexual reproduction; and through these grades they closely approximate each other. For example: sexual reproduction consists essentially in the union of two different cells, the germ-cell and the sperm-cell, to form one cell, the ovum. It is in the most literal sense a union of diverse twain to form one flesh. These two cells may be called the sexual elements. This is all that is absolutely necessary to the idea of sexual reproduction, even though the two elements may be formed by the same organ. But, further, the two elements are usually elaborated by two distinct organs, viz., the ovary and the spermary. These are the essential sexual organs. When these two organs are found in the same individual, the condition is called bisexuality, or hermaphroditism. Further, in the higher animals these two organs exist in different individuals. This condition is called unisexuality. Thus there are several grades of sexuality. The sexual elements only may be separated, or in addition the sexual organs may be separated, or in addition there may be distinct sexual individuals. Any mode of reproduction not answering to this description is non-sexual. But non-sexual reproduction also is of different grades. The lowest is fission. A cell or a community of cells grows and divides itself into two. Each half, again, grows and divides, and so on ad infinitum. Next above this is budding. A spot on the external surface of an organism grows more rapidly than contiguous spots, and forms a tubercle which grows into a bud, assumes the form and structure of the parent, and finally separates. In the next grade the budding is internal, from a special organ simulating an ovary, though not a true ovary, as in aphides. Finally, in parthenogenesis we have a perfect ovary forming true ova and perfect embryo without fertilization or coöperation of the sperm-cell.

Now, my object, more specifically stated, is to show—1, that the highest form, viz., unisexuality, was developed out of bisexuality