按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
are obviously much less marked。 All these small characters； however； are strictly inherited； and this fact makes it very probable that the less obvious constituents of the mixtures in ordinary fields must be constant and pure as long as they do not intercross。 Natural crossing is in most cereals a phenomenon of rare occurrence； common enough to admit of the production of all possible hybrid combinations； but requiring the lapse of a long series of years to reach its full effect。
Darwin laid great stress on this high amount of variability in the plants of the same variety； and illustrated it by the experience of Colonel Le Couteur （〃On the Varieties； Properties； and Classification of Wheat〃； Jersey； 1837。） on his farm on the isle of Jersey； who cultivated upwards of 150 varieties of wheat； which he claimed were as pure as those of any other agriculturalist。 But Professor La Gasca of Madrid； who visited him； drew attention to aberrant ears； and pointed out； that some of them might be better yielders than the majority of plants in the crop； whilst others might be poor types。 Thence he concluded that the isolation of the better ones might be a means of increasing his crops。 Le Couteur seems to have considered the constancy of such smaller types after isolation as absolutely probable； since he did not even discuss the possibility of their being variable or of their yielding a changeable or mixed progeny。 This curious fact proves that he considered the types； discovered in his fields by La Gasca to be of the same kind as his other varieties； which until that time he had relied upon as being pure and uniform。 Thus we see； that for him； the variability of cereals was what we now call polymorphy。 He looked through his fields for useful aberrations； and collected twenty…three new types of wheat。 He was； moreover； clear about one point； which； on being rediscovered after half a century； has become the starting…point for the new Swedish principle of selecting agricultural plants。 It was the principle of single…ear sowing； instead of mixing the grains of all the selected ears together。 By sowing each ear on a separate plot he intended not only to multiply them； but also to compare their value。 This comparison ultimately led him to the choice of some few valuable sorts； one of which； the 〃Bellevue de Talavera；〃 still holds its place among the prominent sorts of wheat cultivated in France。 This variety seems to be really a uniform type； a quality very useful under favourable conditions of cultivation； but which seems to have destroyed its capacity for further improvement by selection。
The principle of single…ear sowing； with a view to obtain pure and uniform strains without further selection； has； until a few years ago； been almost entirely lost sight of。 Only a very few agriculturists have applied it： among these are Patrick Shirreff （〃Die Verbesserung der Getreide…Arten〃； translated by R。 Hesse； Halle； 1880。） in Scotland and Willet M。 Hays （〃Wheat； varieties； breeding； cultivation〃； Univ。 Minnesota； Agricultural Experimental Station； Bull。 no。 62； 1899。） in Minnesota。 Patrick Shirreff observed the fact； that in large fields of cereals； single plants may from time to time be found with larger ears； which justify the expectation of a far greater yield。 In the course of about twenty…five years he isolated in this way two varieties of wheat and two of oats。 He simply multiplied them as fast as possible； without any selection； and put them on the market。
Hays was struck by the fact that the yield of wheat in Minnesota was far beneath that in the neighbouring States。 The local varieties were Fife and Blue Stem。 They gave him； on inspection； some better specimens； 〃phenomenal yielders〃 as he called them。 These were simply isolated and propagated； and； after comparison with the parent…variety and with some other selected strains of less value； were judged to be of sufficient importance to be tested by cultivation all over the State of Minnesota。 They have since almost supplanted the original types； at least in most parts of the State； with the result that the total yield of wheat in Minnesota is said to have been increased by about a million dollars yearly。
Definite progress in the method of single…ear sowing has； however； been made only recently。 It had been foreshadowed by Patrick Shirreff； who after the production of the four varieties already mentioned； tried to carry out his work on a larger scale； by including numerous minor deviations from the main type。 He found by doing so that the chances of obtaining a better form were sufficiently increased to justify the trial。 But it was Nilsson who discovered the almost inexhaustible polymorphy of cereals and other agricultural crops and made it the starting…point for a new and entirely trustworthy method of the highest utility。 By this means he has produced during the last fifteen years a number of new and valuable races； which have already supplanted the old types on numerous farms in Sweden and which are now being introduced on a large scale into Germany and other European countries。
It is now twenty years since the station at Svalof was founded。 During the first period of its work； embracing about five years； selection was practised on the principle which was then generally used in Germany。 In order to improve a race a sample of the best ears was carefully selected from the best fields of the variety。 These ears were considered as representatives of the type under cultivation； and it was assumed that by sowing their grains on a small plot a family could be obtained； which could afterwards be improved by a continuous selection。 Differences between the collected ears were either not observed or disregarded。 At Svalof this method of selection was practised on a far larger scale than on any German farm； and the result was； broadly speaking； the same。 This may be stated in the following words： improvement in a few cases； failure in all the others。 Some few varieties could be improved and yielded excellent new types； some of which have since been introduced into Swedish agriculture and are now prominent races in the southern and middle parts of the country。 But the station had definite aims； and among them was the improvement of the Chevalier barley。 This； in Middle Sweden； is a fine brewer's barley； but liable to failure during unfavourable summers on account of its slender stems。 It was selected with a view of giving it stiffer stems； but in spite of all the care and work bestowed upon it no satisfactory result was obtained。
This experience； combined with a number of analogous failures； could not fail to throw doubt upon the whole method。 It was evident that good results were only exceptions； and that in most cases the principle was not one that could be relied upon。 The exceptions might be due to unknown causes； and not to the validity of the method； it became therefore of much more interest to search for the causes than to continue the work along these lines。
In the year 1892 a number of different varieties of cereals were cultivated on a large scale and a selection was again made from them。 About two hundred samples of ears were chosen； each apparently constituting a different type。 Their seeds were sown on separate plots and manured and treated as much as possible in the same manner。 The plots were small and arranged in rows so as to facilitate the comparison of allied types。 During the whole period of growth and during the ripening of the ears the plots were carefully studied and compared： they were harvested separately； ears and kernels were counted and weighed； and notes were made concerning layering； rust and other cereal pests。
The result of this experiment was； in the main； no distinct improvement。 Nilsson was especially struck by the fact that the plots； which should represent distinct types； were far from uniform。 Many of them were as multiform as the fields from which the parent…ears were taken。 Others showed variability in a less degree； but in almost all of them it was clear that a pure race had not been obtained。 The experiment was a fair one； inasmuch as it demonstrated the polymorphic variability of cereals beyond all doubt and in a degree hitherto unsuspected； but from the standpoint of the selectionist it was a failure。 Fortunately there were； however； one or two exceptions。 A few lots showed a perfect uniformity in regard to all the stalks and ears： these were small families。 This fact suggested the idea that each might have been derived from a single ear。 During the selection in the previous summer； Nilsson had tried to find as many ears as possible of each new type which he recognised in his fields。 But the variability of his crops was so great； that he was rarely able to include more than two or three ears in the same group； and； in a few cases； he found only one representative of the supposed type。 It might； therefore； be possible that those small uniform plots were the direct progeny of ears； the grains of which had not been mixed with those from other ears before sowing。 Exact records had； of course； been kept of the chosen samples； and the number of ears had been noted in each case。 It was； therefore； po