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！
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； possible to answer the question and it was found that those plots alone were uniform on which the kernels of one single ear only had been sown。 Nilsson concluded that the mixture of two or more ears in a single sowing might be the cause of the lack of uniformity in the progeny。 Apparently similar ears might be different in their progeny。
Once discovered； this fact was elevated to the rank of a leading principle and tested on as large a scale as possible。 The fields were again carefully investigated and every single ear； which showed a distinct divergence from the main type in one character or another； was selected。 A thousand samples were chosen； but this time each sample consisted of one ear only。 Next year； the result corresponded to the expectation。 Uniformity prevailed almost everywhere； only a few lots showed a discrepancy； which might be ascribed to the accidental selection of hybrid ears。 It was now clear that the progeny of single ears was； as a rule； pure； whereas that of mixed ears was impure。 The single…ear selection or single…ear sowing； which had fallen into discredit in Germany and elsewhere in Europe； was rediscovered。 It proved to be the only trustworthy principle of selection。 Once isolated； such single…parent races are constant from seed and remain true to their type。 No further selection is needed； they have simply to be multiplied and their real value tested。
Patrick Shirreff； in his early experiments； Le Couteur； Hays and others had observed the rare occurrence of exceptionally good yielders and the value of their isolation to the agriculturist。 The possibility of error in the choice of such striking specimens and the necessity of judging their value by their progeny were also known to these investigators； but they had not the slightest idea of all the possibilities suggested by their principle。 Nilsson； who is a botanist as well as an agriculturist； discovered that； besides these exceptionably good yielders； every variety of a cereal consists of hundreds of different types；