=186. Variation and mutation.= New forms of plants are known to arise by three methods, viz., variation, mutation, adaptation. The evidence in support of these is almost wholly observational, and consequently more or less inexact, but for each there exist a few accurate experiments which are conclusive. Origin by variation and subsequent selection is the essence of the Darwinian theory of the origin of species. According to this the appearance of a new form is due to the accumulation, and selection, through a long period, of minute differences which prove advantageous to the plant in its competition with others in nature, or are desirable under cultivation. Slight variations appear indiscriminately in every species. Their cause is not known, but since they are found even in the most uniform habitats, it is impossible to find any direct connection between them and the physical factors. In the case of origin by mutation, the new form appears suddenly, with definite characteristics fully developed. Selection, in the usual sense of the term, does not enter into mutation at all, though the persistence of the new form is still to be determined by competition. Mutations are known at present for only a few species, and their actual appearance has been studied in a very few cases. Like variations, they are indiscriminate in character. The chief difference between them is apparently one of degree. Indeed, mutation lends itself readily to the hypothesis that it is simply the sudden appearance of latent variations which have accumulated within the plant. De Vries regards constancy as an essential feature of mutation, but the evidence from the mutants of _Onagra_ is not convincing. Indeed, while there can be no question of the occurrence of mutation in plants, a fact known for many years, the facts so far brought forward in support of the “mutation theory” fall far short of proving “the lack of significance of individual variability, and the high value of mutability for the origin of species.”[19] Mutations do not show any direct connection with the habitat, but their sudden appearance suggests that they may be latent or delayed responses to the ordinary stimuli. Origin by adaptation is the immediate consequence of the stimuli exerted by the physical factors of a habitat. This fact distinguishes it from origin by variation, or by mutation. The new form may appear suddenly, often in a single generation, or gradually, but in either case it is the result of adaptation that is necessarily advantageous, because it is the result of adjustment to controlling physical factors. Origin by adaptation is perhaps only a special kind of origin by variation, but this might be said with equal truth of mutation. New forms resulting from adaptation are like those produced from mutation, in that they appear suddenly as a rule and without the agency of selection. They are essentially different, inasmuch as their