The mutation theory stands in sharp contrast to the selection theory. The latter uses as its starting-point the common form of variability known as individual or fluctuating variation; but according to the mutation theory there are two kinds of variation that are entirely different from each other. “The fluctuating variation can, as I hope to show, not overstep the bounds of the species, even after the most prolonged selection,—much less can this kind of variation lead to the production of new, constant characters.” Each peculiarity of the organism has arisen from a preceding one, not through the common form of variation, but through a sudden change that may be quite small but is perfectly definite. This kind of variability that produces new species, De Vries calls mutability; the change itself he calls a mutation. The best-known examples of mutations are those which Darwin called “single variations” or “sports.”
De Vries recognizes the following kinds of variation:—
First, the polymorphic forms of the systematists. The ordinary groups which, following Linnæus, we call species, are according to De Vries collective groups, which are the outcome of mutations. Many such Linnæan species include small series of related forms, and sometimes even large numbers of such forms. These are as distinctly and completely separated from each other as are the best species. Generally these small groups are called varieties, or subspecies,—varieties when they are separated by a single striking character, subspecies when they differ in the totality of their characters, in the so-called habitus.
These groups have already been recognized by some investigators as elementary species, and have been given corresponding binary names. Thus there are recognized two hundred elementary species of the form formerly called Draba verna.
When brought under cultivation these elementary species are constant in character and transmit their peculiarities truly. They are not local races in the sense that they are the outcome in each generation of special external conditions. Many other Linnæan species are in this respect like Draba verna, and most varieties, De Vries thinks, are really elementary species.
Second, the polymorphism due to intercrossing is the outcome of different combinations of hereditary qualities. There are here, De Vries says, two important classes of facts to be kept strictly apart,—scientific experiment, and the results of the gardener and of the cultivator. The experimenter chooses for crossing, species as little variable as possible; the gardener and cultivator on the other hand prefer to cross forms of which one at least is variable, because the variations may be transmitted to the hybrid, and in this way a new form be produced.
New elementary characters arise in experiments in crossing only through variability, not through crossing itself.
Third, variability in the ordinary sense, that is, individual variability, includes those differences between the individual organs that follow Quetelet’s theory of chance. This kind of variability is characterized by its presence at all times, in all groups of individuals.
De Vries recalls Galton’s apt comparison between variability and a polyhedron which can roll from one face to another. When it comes to rest on any particular face, it is in stable equilibrium. Small vibrations or disturbances may make it oscillate, but it returns always to the same face. These oscillations are like the fluctuating variations. A greater disturbance may cause the polyhedron to roll over on to a new face, where it comes to rest again, only showing the ever present fluctuations around its new centre. The new position corresponds to a mutation. It may appear from our familiarity with the great changes that we associate with the idea of discontinuous variability, that a mutation must also involve a considerable change. Such, however, De Vries says, is not the case. In fact, numerous mutations are smaller than the extremes of fluctuating variation. For example, the different elementary species of Draba verna are less different from each other than the forms of leaves on a tree. The essential differences between the two kinds of variation is that the mutation is constant, while the continuous variation fluctuates back and forth.
The following example is given by De Vries to illustrate the general point of view in regard to varieties and species. The species Oxalis corniculata is a “collective” species that lives in New Zealand. It has been described as having seven well-characterized varieties which do not live together or have intermediate forms. If we knew only this group, there would be no question that there are seven good species. But in other countries intermediate forms exist, which exactly bridge over the differences between the seven New Zealand forms. For this reason all the forms have been united in a single species.