Whether the pollen of a flower is utilized in one or other of its thousands of pollen-grains by reaching the stigma of another plant of the same species, or whether all the pollen-grains are uselessly scattered abroad, cannot possibly affect the flower so as to cause degeneration; the theory of disuse cannot be applied in this case. What is true of the potato holds good also of the manioc (_Manihot utilissima_), but, on the other hand, many of the best varieties of common fruits--pears, figs, grapes, pine-apples, and bananas--are seedless. In _Maranta arundinacea_ 'the whole wonderful structure of the flower has persisted, but the pollen-grains, that is the germ-cells, are wanting.' Whether this implies a permanent degeneration of the sexual organs, that is to say, one that is embodied in the primary constituents of the species, or whether it is only the result of over-abundant nourishment, or of other causes in the circumstances affecting the particular plant, can only be decided by experiment. Probably both occur. The common ivy, for instance, does not now blossom in the northern parts of Sweden and Russia, but it does so still in the southern provinces. If plants were brought to us from the most northerly zone of distribution, they would in all probability flower and bear fruit with us, and in that case the absence of bloom in these plants must have been a direct effect of the cold climate. But it is quite conceivable that cultivated plants have in many cases become hereditarily infertile, when they are constantly propagated only by means of buds, layering, and so on, not however because of any direct effect of this mode of propagation, but through chance germinal variations. For in regard to many of them man has lost all interest in the flowers and fruit, as, for instance, in the case of the potato; in other cases he is even interested in procuring seedless fruits. In the first case he will quite readily make use of plants with imperfect flowers for propagating, if they are otherwise fit and exhibit what he wants in other respects; in the second case, he will give a preference to individuals with seedless fruits, and thus increase and strengthen the tendency to degeneration of the seeds in the race concerned.