The order in which our subject will be treated in the present volume is as follows. A long series of experiments will first be given in Chapters 2 to 6. Tables will afterwards be appended, showing in a condensed form the relative heights, weights, and fertility of the offspring of the various crossed and self-fertilised species. Another table exhibits the striking results from fertilising plants, which during several generations had either been self-fertilised or had been crossed with plants kept all the time under closely similar conditions, with pollen taken from plants of a distinct stock and which had been exposed to different conditions. In the concluding chapters various related points and questions of general interest will be discussed. Anyone not specially interested in the subject need not attempt to read all the details (marked []); though they possess, I think, some value, and cannot be all summarised. But I would suggest to the reader to take as an example the experiments on Ipomoea in Chapter 2; to which may be added those on Digitalis, Origanum, Viola, or the common cabbage, as in all these cases the crossed plants are superior to the self-fertilised in a marked degree, but not in quite the same manner. As instances of self-fertilised plants being equal or superior to the crossed, the experiments on Bartonia, Canna, and the common pea ought to be read; but in the last case, and probably in that of Canna, the want of any superiority in the crossed plants can be explained.