I may perhaps be allowed to avail myself of this opportunity to point out a practical side of the study of elementary species. This always appears whenever wild plants are subjected to cultivation, either in order to reproduce them as pure strains, or to cross them with other already cultivated species. The latter practice is as a rule made use of whenever a wild species is found to be in possession of some quality which is considered as desirable for the cultivated forms. In the case of the beach-plum it is the hardiness and the great abundance of fruits of the wild species which might profitably be combined with the recognized qualities of the ordinary plums. Now it is manifest, that in order to make crosses, distinct individual plants are to be chosen, and that the variability of the wild species may be of very great importance. [59] Among the range of elementary species those should be used which not only possess the desired advantages in the highest degree, but which promise the best results in other respects or their earliest attainment. The fuller our knowledge of the elementary species constituting the systematic groups, the easier and the more reliable will be the choice for the breeder. Many Californian wild flowers with bright colors seem to consist of large numbers of constant elementary forms, as for instance, the lilies, godetias, eschscholtias and others. They have been brought into cultivation many times, but the minutest distinction of their elementary forms is required to attain the highest success. In concluding, I will point out a very interesting difficulty, which in some cases impedes the clear understanding of elementary species. It is the lack of self-fertilization. It occurs in widely distant families, but has a special interest for us in two genera, which are generally known as very polymorphous groups.