But the current belief is, that they are only true to their peculiarities to a certain degree, and that from time to time, and not rarely, they revert to the type from which they have arisen. Such reversion is supposed to prove that they are mere varieties, and at the same time to indicate empirically the species from which they have sprung. In the next lecture we shall examine critically the evidence on which this assumption rests. Before doing so however, it will be necessary to collate the cases in which there is no reversion at all, or in which the reversion is absent at least in experimental and pure sowings. In the present state of our knowledge it is very difficult to decide, whether or not true reversion occurs in constant varieties. If it does occur, it surely does so very rarely and only under unusual circumstances, or in particular individuals. However when such individuals are multiplied by buds and especially when they are the only representatives of their type, the reversion, though theoretically rare, will be shown by nearly every specimen of the variety. Examples of this will be given below. [156] They are generally called atavists or reversionists, but even these terms are sometimes used in a different sense. Lastly it is to be said that the empirical and experimental evidence as to the question of constancy is not as extensive as it should be. The experimental conditions are seldom described, and it is only recently that an interest in the matter has been awakened. Much remains to be done. Among other things the innumerable varieties of trees, shrubs and perennial herbs should be tested as to their constancy when grown from purely fertilized seeds. Many of them may be included among the number that sport constantly. Leaving aside the doubtful or insufficiently studied cases, we may now turn our attention to the facts that prove the absolute stability of a large number of varieties, at least as far as such completeness can be attained by experiment or observation.