It may be well to stop here, to show the entire difference between this and non-sexual modes. The latter, we have seen, is only a modification of growth, an extension of the individual. Now, sexual reproduction is the opposite of all this. Growth is a constant multiplication of cells. One cell is ever becoming two similar cells--or, if we call them individuals, one individual is ever becoming two similar individuals. But in sexual reproduction we have an exactly reverse process. Reduced to its simplest terms, sexual reproduction is the fusion of _two diverse_ cells, sperm-cell and the germ-cell, to form _one_ cell, the ovule--literally, a diverse twain forming one flesh. In its higher forms it is the union of diverse _individuals_ to bring about the same result. Instead of one cell becoming two, it is two cells becoming one; instead of one individual becoming two in the offspring, it is two individuals becoming one in the offspring. But this great change was not brought about at once, but only in the most gradual manner. First, the sexual elements--sperm-cell and germ-cell--are separated, but _in the same organ_. Then the organs--spermary and ovary--are separated, but in the _same individual_. This is the condition of self-fertilizing hermaphroditism so common among plants and lower animals. Then comes cross-fertilizing hermaphroditism; and Nature takes much pains and uses many ingenious devices to prevent self-fertilization and insure cross-fertilization. Now, for the first time, we have slight individual differences funded in a common offspring. Then, in order to absolutely forbid self-fertilization, and at the same time allow greater differences in the crossing individuals than could be attained in hermaphroditic individuals, the sex organs are separated in _different individuals_, and fertilization can only take place by _voluntary union_. Then, to insure the union of suitable individuals, and forbid the ban between unsuitable, there are introduced sexual attraction and repulsion. Then, last of all, the difference between the two sex-individuals becomes greater and greater as we go up. It is conspicuous only in vertebrates and some insects, and very conspicuous only in birds and mammals.