Seeds often lie dormant for several years in the ground, and germinate when brought near the surface by any means, as by burrowing animals. They would probably be affected by the mere circumstance of having long lain dormant; for gardeners believe that the production of double flowers, and of fruit, is thus influenced. Seeds, moreover, which were matured during different seasons will have been subjected during the whole course of their development to different degrees of heat and moisture. It has been shown that pollen is often carried by insects to a considerable distance from plant to plant. Therefore, one of the parents or ancestors of our two plants of _Digitalis_ may have been crossed by a distant plant growing under somewhat different conditions. Plants thus crossed often produce an unusually large number of seeds; a striking instance of this fact is afforded by the _Bignonia_, which was fertilized by Fritz Müller with pollen from some adjoining plants and set hardly any seed, but, when fertilized with pollen from a distant plant, was highly fertile. Seedlings from a cross of this kind grow with great vigor, and transmit their vigor to their descendants. These, therefore, in the struggle for life, will generally beat and exterminate the seedlings from plants which have long grown near together under the same conditions, and will thus tend to spread. PRACTICAL APPLICATION OF THESE VIEWS. [Page 458.] Under a practical point of view, agriculturists and horticulturists may learn something from the conclusions at which we have arrived. Firstly, we see that the injury from the close breeding of animals and from the self-fertilization of plants does not necessarily depend on any tendency to disease or weakness of constitution common to the related parents, and only indirectly on their relationship, in so far as they are apt to resemble each other in all respects, including their sexual nature. And, secondly, that the advantages of cross-fertilization depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, or from their having intercrossed with individuals thus exposed; or, lastly, from what we call in our ignorance spontaneous variation. He therefore who wishes to pair closely related animals ought to keep them under conditions as different as possible. * * * * * [Page 459.] As some kinds of plants suffer much more from self-fertilization than do others, so it probably is with animals from too close interbreeding. The effects of close interbreeding on animals, judging again from plants, would be deterioration in general vigor, including fertility, with no necessary loss of excellence of form; and this seems to be the usual result.