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Atavism and vicinism are both cases of variability, but in wholly different sense. For this reason it may be as well, to insert here a short survey of the general meanings to be conveyed by the term variation. It implies in the first place the occurrence of a wide range of forms and types, irrespective of their origin, and in the second place the process of the change in such forms.
In the light of this experience, Metzger's observation becomes a typical instance of vicinism. It relates to the "Tuscarora" corn of St. Louis, a variety with broad and flat white seeds. About the year 1840, this corn was introduced into Baden in Germany, and cultivated by Metzger.
You will be led to conclude that you are observing a sudden change in structure such as is usually called a sport. Or in other words you may think that you are assisting at the origination of a new variety. If you are familiar with the principle of vicinism, you will refrain from such an inference and consider the supposition of a hybrid origin.
We may infer therefore that the word variability has such a wide range of meanings that it ought never be used without explanation. Nothing indeed, is more variable than the signification of the term variable itself. For this reason, we will furthermore designate all variations under the influence of neighbors with the new and special term "vicinism." It always indicates the result of crossing.
And this mode of cultivation being in almost universal use in the larger nurseries, we may, by this discussion, arrive at a more scientific estimation of the phenomena of vicinism, hitherto described. The simplest case that may be given, is when an ordinary retrograde variety is cultivated with the species to which it belongs.
This impurity we have called vicinism, and in a previous lecture have shown its effects upon the horticultural races on one hand, and on the other, on the scientific value that can be ascribed to the experience of the breeder. We have established the general rule that stability is seldom met with, but that the observed instability is always open to the objection of being the result of vicinism.
With Chrysanthemum coronarium and blue-bottles this figure is often announced to be only about 50%. No doubt it is partly due to impurities, caused by vicinism, but it is obviously improbable that the effect of these impurities should be so large. Some cases of partial reversion may be interpreted in the same way.
Apetalous flowers may be considered as another form of monstrosity, and in Salpiglossis sinuata such a variety without a corolla made its appearance in the year 1892 in the nursery of Vilmorin. It appeared suddenly, yielded a good crop of seed and was constant from the outset, without any sign of vicinism or impurity.
But in former times, when this principle was still unknown and not even guessed at, it is evident that many mistakes must have been made, and that many an instance, which until now has been considered reliable proof of a so-called single variation, is in fact only a case of vicinism. In reading the sparse literature on sports, numerous cases will be found, which cannot stand this test.
Cut-leaved walnuts have been known since 1812; they come true from seed, but are extremely liable to vicinism, a nuisance which is ascribed by some authors to the fact that often on the same tree the male catkins flower and fall off several weeks before the ripening of the pistils of the other form of flowers. Weeping varieties afford similar instances.
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