Mutation Theory (1908)
Hugo de Vries
§ 11. SUGAR BEETS.
Sugar beets afford the finest example of the process of artificial selection. In no other plant under cultivation has the technique of selection reached so high a pitch of perfection; in no other is the method so sure or the result so certain. There is now no sale for beet seed which has not been the result of careful selection.
Experiments in the selection of sugar capacity began about 1850. This instance shows best, therefore, what can be achieved within half a century by continued selection in one and the same direction, hand in hand with continual improvement of method.
Progress has been enormous: the average content of the common beet, which at first was a matter of 7-8%, is now double that amount. Shape, size, and weight, the character of the leaves and especially the reduction in woody tissues have all been the object of selection, and have made the beet much more valuable from the industrial point of view.
All this has been done by selection of the best individuals afforded by ordinary fluctuating variation. Neither spontaneous variations nor crossings have played any part in it. We are dealing here with the process in its simplest form.
|1I particularly recommend to the scientific reader the study of Prof. KURT VON RÜMKER'S short and clear paper: Die Zuckerrübenzüchtung der Gegenwart. (Blätter für Zuckerrübenbau, 1894, pp. 1-48.)|
This is not the place to praise the genius of LOUIS VILMORIN, the founder of the method, or the achievements of his numerous successors especially in Germany. Nor need I describe the marvelous technical process by which it is possible to determine the polarization indices of more than 100,000 beets in a few weeks.1
On the contrary I am only concerned with showing how little value these splendid results have in the discussion of the process of the origin of species. On the botanical side no better argument for the theory of selection could be adduced. Yet in this case there is nothing which is in the remotest degree like the origin of a new species nor even anything that could lead us to expect that any form of the systematic value of a species could arise in this way.
|1PLINIUS, N. H. Lib. 19. See also COLUMELLA and CICERO (Note of 1908).|
Of course I am not speaking of the origin of the sugarbeet itself. We know as little of its origin as we do of the origin of the other varieties of beets. The Romans probably had only two sorts which they used as vegetables, which they cultivated in their gardens and collected in the wild state.1 At the beginning of the nineteenth century there were numerous kinds. The question arises: did they originate from the older forms in culture, or were they first found as distinct subspecies in nature? We do not know. That they had a common origin we do not doubt, but whether they originated before or during cultivation remains a mystery.
The beet with which VILMORIN began his work half a century ago must be regarded simply as a starting point; artificial selection is responsible only for what it has given rise to.
|2L. LÉVÊQUE DE VILMORIN, Notices sur l'amélioration des plantes par le semis. 2d edition, 1886; see especially p. 27.|
As far back as 1830-1840 VILMORIN had selected his beets according to their external form. In 1851 he determined the saccharine contents of single roots and found that it varied from 7-14%, but the troublesome nature of the methods of estimation available at that time only permitted of the determination of comparatively few instances. He discovered the best beets by their high specific gravity in salt solutions, sowed the seeds which they produced and got beets with 21% sugar in the second generation.2
These figures (7-14-21%) are very important in this connection. It must be remembered of course that they cannot be compared very exactly with the results of recent work because the method has become much more simple and precise, especially since the introduction of the use of the polariscope.
But it is more likely that VILMORIN'S figures were too low than that they were too high.
|1LANGETHAL, Landwirthschaftliche Pflanzenkunde, III, 1874, p. 69.
2Jahresbericht der Zuckerindustrie, Vol. 9, p. 39 etc.
1874 was the first year in which the method of polarization was employed and the selection based on the results of this method. The normal contents ranged at that time between 10-14%. In bad years with a mean of 10%; in good ones it was from 12 to 14%.1 Even cases of 9.5 and 17.5% were not rare.2 From 1878 to 1881 the method of polarization spread in Germany and Austria; I need only mention the names of DIPPE of Quedlinburg, RIMPAU, HEINE and the Klein-Wanzleben factory. The progress was slow but sure. In most factories the beets are examined only comparatively, except in the case of the best ones, in which the polarization index was actually determined. In the works of Messrs. KUHN & Co., however, at Naarden (Holland) this index has been directly determined every year for over 300,000 plants. Through the kindness of these gentlemen I obtained in 1896 the indices of 40,000 roots; they made a very beautiful curve with a mean at 15.5% (Fig. 22.)
Selection is then carried out with reference to these figures in such a way that sufficient plants are always available for seed. The result of polarization determines the limits of the groups. I shall now give some figures for 1892. Roots with less than 14% were not planted for seed. Those with from 14-16% formed the seed plants for commercial seed; there were 20 to 30 of these in every 100 plants examined. Those with from 16-18% became the seed plants for the special race, the so-called elite race from whose seeds the beets were produced which would be tested in the next generation. In 1892 there were among 180,000 polarizations only four instances of a higher percentage than 18% in the above mentioned factory. Since that time the annual number of polarizations as already mentioned has reached about 300,000; and the maximum percentage has mounted to 21%, and the other figures have risen correspondingly. Commercial beet grown from this selected seed, has on the average 13 to 14% sugar.
Fig. 22. Variation in the Amount of Sugar in 40,000 Beets 1
|1The polarizations in question were carried out from January 2 to February 5 in 1896 and gave the following figures:|
|Individuals with less than 12% have been excluded from this series. The dotted line is the theoretical curve according to QUETELET'S law, (a+b)20; the discrepancy between it and the actual curve on the left side is probably due to the presence of some faulty beets.|
Without underrating the high agricultural significance of these results we must nevertheless face the fact that there is very little in them which can serve as a basis for a decision of the question of the origin of specific characters. We cannot be certain whether after selection for fifty years, i. e., for 25 generations, the upper limit of the range of variability has been essentially extended or not. It so happens that this limit — 21 % — is the same in VILMORIN'S instance (1853) and in the works at Naarden (1892-1898), but there are other races under different conditions of cultivation, whose limit is stated to be as high as 26%. The wider range of modern polarization work with beets evidently gives a chance of higher percentages.
The average product of a field has certainly increased from 7-8% to 14-16% and more. But this improvement is dependent on the continuation of selection; and it is only by this means that it has reached this pitch. Every sugar manufacturer knows that selection is an indispensable condition of a satisfactory harvest. It is true that, for the purpose of obtaining the necessary quantity of seed, a so-called intermediate generation is interpolated between polarization and seed-harvest: but if more than one or at most two (and this happens very seldom) of these are introduced the advantage gained by polarization and selection is lost. By no manner of means is the improvement independent of selection; on the contrary the promise of more sugar can only be fulfilled by a further perfection of the method of polarization and by continued efforts on the part of breeders.
Since the method of polarization was introduced the progress of the sugar beet has been slow but continuous. But it must not be concluded from this that the present maximum could not have been reached in a few generations. At any rate we are not justified in deriving this conclusion from the evidence at hand. Progress has obviously been due to the steady improvement in methods of selection. This has consisted first in the invention of the boring cylinder which enables us to polarize directly the beets to be selected; VILMORIN and his immediate followers had to sacrifice the whole beet to their chemical analysis, and then to select others, resembling the best ones in specific gravity, for cultivation. And secondly by employing larger and larger groups to choose from, (in the best factories every year more than 100,000 beets). For it is evident that the larger number there is to choose from, the greater chance is there of finding desirable ones.
|1v. RÜMKER, Zuckerrübenzüchtung, p. 5.|
Beets have been selected not merely with regard to their saccharine contents but also with regard to their external features. This takes place in the field at the time of harvest, that is to say, before polarization. In most factories about are thrown away in this process and only 1/10 saved. Breeders are of the opinion that on the whole this 9/10 plants inferior in respect to their saccharine contents and that by this selection a beneficial effect on the percentage of sugar itself is brought about.1 In this preliminary selection attention is paid first to the leaves; the features dealt with being their shape, their size and the angle which they make with the zenith, as well as the general features which control assimilation, transpiration and the non-retention of rain water. The various kinds of beet, in which selection has had different objects in view, can be recognized by their foliage in the field. The form of the root is very important; it should be unbranched; the more like the roots are to one another the more easily can they be dealt with. The dimensions of the stem, or the head as it is called, and many other points have all to be considered and especially the size or the weight, of the whole beet.
Individual breeders pay attention to trivial characters as for example the red color in the seedling with the object of facilitating the detection of impurities in their strains in the field.
It is absolutely essential to keep one's eye on all these points in every generation. In the case of no single character can selection be relaxed. Any disregard of these rules on the part of the breeder would soon lead to a degeneration of the whole race.
|1VON RÜMKER, Der Landwirthschaftliche Mehrwerth, loc. cit., p. 136.|
"Each race of plant possesses only a very small degree of constancy." Herein lies the difference between the improved race and the species. This already quoted expression of VON RÜMKER1 sums up clearly and concisely the whole significance of agricultural results in their bearing on Natural Selection.