the sterility of species when first crossed and of their hybrid offspring. It was this consideration which led me to make the many observations recorded in this chapter, and which in my opinion make them worthy of publication. CHAPTER VI. CONCLUDING REMARKS ON HETEROSTYLED PLANTS. The essential character of heterostyled plants. Summary of the differences in fertility between legitimately and illegitimately fertilised plants. Diameter of the pollen-grains, size of anthers and structure of stigma in the different forms. Affinities of the genera which include heterostyled species. Nature of the advantages derived from heterostylism. The means by which plants became heterostyled. Transmission of form. Equal-styled varieties of heterostyled plants. Final remarks. In the foregoing chapters all the heterostyled plants known to me have been more or less fully described. Several other cases have been indicated, especially by Professor Asa Gray and Kuhn, in which the individuals of the same species differ in the length of their stamens and pistils (6/1. Asa Gray ‘American Journal of Science’ 1865 page 101 and elsewhere as already referred to. Kuhn ‘Botanische Zeitung’ 1867 page 67.); but as I have been often deceived by this character taken alone, it seems to me the more prudent course not to rank any species as heterostyled, unless we have evidence of more important differences between the forms, as in the diameter of the pollen-grains, or in the structure of the stigma. The individuals of many ordinary hermaphrodite plants habitually fertilise one another, owing to their male and female organs being mature at different periods, or to the structure of the parts, or to self-sterility, etc.; and so it is with many hermaphrodite animals, for instance, land-snails or earth-worms; but in all these cases any one individual can fully fertilise or be fertilised by any other individual of the same species. This is not so with heterostyled plants: a long-styled, mid-styled or short-styled plant cannot fully fertilise or be fertilised by any other individual, but only by one belonging to another form. Thus the essential character of plants belonging to the heterostyled class is that the individuals are divided into two or three bodies, like the males and females of dioecious plants or of the higher animals, which exist in approximately equal numbers and are adapted for reciprocal fertilisation. The existence, therefore, of two or three bodies of individuals, differing from one another in the above more important characteristics, offers by itself good evidence that the species is heterostyled. But absolutely conclusive evidence can be derived only from experiments, and by finding that pollen must be applied from the one form to the other in order to ensure complete fertility.