The means to attract insects are threefold: nectar, odor and color. The nectar is a sweet liquid which the flower secretes by means of nectar glands. The latter are usually situated near the base of the flower. Other plants attract insect visitors by giving up a sweet scent. These are especially the small flowers, like the mignonette or evening primrose. The color is another means by which the flower attracts insects and birds. The color of the flower is, as a rule, due to showy petals. Different kinds of insects are especially attracted by different colors. Some flowers with very long tubulated corollas depend entirely upon birds with long beaks to carry their pollen for them. In complete and perfect flowers, where stamen and pistil are present in the same flower, self-fertilization would be the rule, if there were not certain means for its prevention. In the first place the pollen of another plant frequently prevails over that which the flower may shed over its own pistil. When both kinds are placed over the stigma, at the same time, it is the foreign pollen which causes fertilization. Another means to prevent self-fertilization consists in the stamens and pistils maturing at different times. The insect visitor, on its way to the nectary, brushes against the ripe stamens of a certain flower in its earlier stage. It cannot deposit the acquired pollen upon the stigma of the same flower and thus cause self-fertilization because the pistil is not ripe yet. But, in flying to a flower in the later stage, when the stigma has already ripened and the stamens have shed all their pollen, the insect will lodge the pollen first acquired on the ripe stigma, and in this way produce the desired cross-fertilization. _Sex-differentiation._—When we rise higher in the scale of animal life we find that the egg-cells and sperm-cells are almost always produced by different individuals. Those which produce egg-cells or ova are called female, and those which produce sperm-cells or spermatozoa are called male animals. The formation of a new being results in these species from the conjugation of two cellular elements of two different animals. In this way the origin of the organism, or the zygote from which the new individual develops is composed of parts of two different individuals, and a difference between the offspring and parents is insured. The intermixing of body-substance from two distinct individuals and the development therefrom of the new individual produce variation.[Z]