Naturalists are so much accustomed to behold great diversities of structure associated with the two sexes, that they feel no surprise at almost any amount of difference; but differences in sexual nature have been thought to be the very touchstone of specific distinction. We now see that such sexual differences--the greater or less power of fertilising and being fertilised--may characterise the co-existing individuals of the same species, in the same manner as they characterise and have kept separate those groups of individuals, produced during the lapse of ages, which we rank and denominate as distinct species. CHAPTER VII. POLYGAMOUS, DIOECIOUS, AND GYNO-DIOECIOUS PLANTS. The conversion in various ways of hermaphrodite into dioecious plants. Heterostyled plants rendered dioecious. Rubiaceae. Verbenaceae. Polygamous and sub-dioecious plants. Euonymus. Fragaria. The two sub-forms of both sexes of Rhamnus and Epigaea. Ilex. Gyno-dioecious plants. Thymus, difference in fertility of the hermaphrodite and female individuals. Satureia. Manner in which the two forms probably originated. Scabiosa and other gyno-dioecious plants. Difference in the size of the corolla in the forms of polygamous, dioecious, and gyno-dioecious plants. There are several groups of plants in which all the species are dioecious, and these exhibit no rudiments in the one sex of the organs proper to the other. About the origin of such plants nothing is known. It is possible that they may be descended from ancient lowly organised forms, which had from the first their sexes separated; so that they have never existed as hermaphrodites. There are, however, many other groups of species and single ones, which from being allied on all sides to hermaphrodites, and from exhibiting in the female flowers plain rudiments of male organs, and conversely in the male flowers rudiments of female organs, we may feel sure are descended from plants which formerly had the two sexes combined in the same flower. It is a curious and obscure problem how and why such hermaphrodites have been rendered bisexual. If in some individuals of a species the stamens alone were to abort, females and hermaphrodites would be left existing, of which many instances occur; and if the female organs of the hermaphrodite were afterwards to abort, the result would be a dioecious plant. Conversely, if we imagine the female organs alone to abort in some individuals, males and hermaphrodites would be left; and the hermaphrodites might afterwards be converted into females.