So much, then, for the practical difference between a mutation and a fluctuating variation. In Chapter V. we shall discuss the possible causes of the difference. By way of anticipation we may say that the suggestion we shall make is that a mutation is due to some rearrangement in the particles which represent that part of the organism in the fertilised egg, whereas a fluctuating variation is caused by variations in the particles themselves.
De Vries, it should be noted, bases his theory largely on experimental evidence. His dictum is “the origin of species is an object of experimental observation.” He has, we consider, proved conclusively that among plants mutations sometimes occur, and, further, that in a mutating plant the same mutation tends to occur again and again. This latter is a most important fact, because it goes some way towards overcoming the difficulty urged by Darwin that isolated sports must be swamped by continual crossing with the normal type. If mutations arise in swarms, as De Vries asserts they do, then any particular mutation is likely, sooner or later, to cross with a similar mutation and so be able to perpetuate itself.
Mutating Plants
The classical example of a mutating plant is the evening primrose of the species Oenothera lamarckiana. This is described by De Vries as a stately plant, with a stout stem, attaining often a height of 1.6 metres or more. The flowers are large and of a bright yellow colour, attracting immediate attention, even from a distance. “This striking species,” he writes, in Species and Varieties (p. 525), “was found in a locality near Hilversum, in the vicinity of Amsterdam, where it grew in some thousands of individuals. Ordinarily biennial, it produces rosettes in the first, and stems in the second year. Both the stems and the rosettes were seen to be highly variable, and soon distinct varieties could be distinguished among them.
“The first discovery of this locality was made in 1886. Afterwards I visited it many times, often weekly or even daily, and always at least once a year up to the present time. This stately plant showed the long-sought peculiarity of producing a number of new species every year. Some of them were observed directly in the field, either as stems or rosettes. The latter could be transplanted into my garden for further observation, and the stems yielded seeds to be sown under like control. Others were too weak to live a sufficiently long time in the field. They were discovered by sowing seed from indifferent plants of the wild locality in the garden. A third and last method of getting still more new species from the original strain was the repetition of the sowing process, by saving and sowing the seed which ripened on the introduced plants. These various methods have led to the discovery of over a dozen new types, never previously observed or described.” Some of these De Vries regards as varieties, in the sense in which he uses the words; others, he maintains, are real progressive species, some of which are strong and healthy, others weaker and apparently not destined to be successful. All these types proved absolutely constant from seed. “Hundreds of thousands of seedlings may have arisen, but they always come true and never revert to the original O. lamarckiana type. But some of them, however, are, like their parent form, liable to mutations.” The case of the evening primrose is by no means an isolated one. De Vries cites several other instances of plants in a mutating state. “The common poppy,” he says (p. 189), “varies in height, in colour of foliage and flowers; the last are often double or laciniated. It may have white or bluish seeds, the capsules may open themselves or remain closed, and so on. But every single variety is absolutely constant, and never runs into another when the flowers are artificially pollinated and the visits of insects excluded.” Similarly the garden carnation sometimes gives rise to the wheat-ear form. “In this variety,” writes De Vries (p. 228), “the flower is suppressed, and the loss is attended by a corresponding increase in the number of pairs of bracts. This malformation results in square spikes, or somewhat elongated heads, consisting only of the greenish bracts. As there are no flowers, the variety is quite sterile, and, as it is not regarded by horticulturists as an improvement on the ordinary bright carnations, it is seldom multiplied by layering. Notwithstanding this it appears from time to time, and has been seen in different countries and at different periods, and what is of great importance for us, in different strains of carnations. Though sterile, and obviously dying out as often as it springs into existence, it is nearly two centuries old. It was described in the beginning of the eighteenth century by Volckamer, and afterwards by Jaeger, De Candolle, Weber, Masters, Magnus, and many other botanists. I have had it twice at different times and from different growers.” Similarly, the long-headed green dahlia arose twice over some years ago in the nursery of Messrs Zocher & Co.
Further, the peloric Toad-flax (Linaria vulgaris peloria) is, De Vries informs us, “known to have originated from the ordinary type at different times and in different countries under more or less divergent conditions.” And, as this variety is wholly barren, it must in each instance have had an independent origin. Lastly, the purple beech seems to be a mutation which has originated at least three times over.
Mutation Theory Criticised
Every one interested in biological theory should read both Species and Varieties and Plant Breeding by De Vries, works which are of incalculable value to the horticulturist and agriculturist as well as to the biologist.
While not wishing to detract in any way from the truly splendid work done by De Vries, we feel constrained to bring several charges against him.
Firstly, he suffers from the complaint that seizes nine out of ten originators of new theories. He pushes his theory to extreme lengths; he allows his imagination to run away with him. We do not think that on the evidence available he is justified in asserting that every species passes through alternating periods of comparative quiescence and periods in which it throws off, as mutations, swarms of elementary species. He is justified in asserting that discontinuous variation is by no means an uncommon phenomenon, but further than this it does not seem safe to go at present.