ed 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； which find the best conditions for success when grown together； but which； after isolation； prove to be constant。 Their preference for mixed growth is so definite； that once isolated； their claims on manure and treatment are found to be much higher than those of the original mixed variety。 Moreover； the greatest care is necessary to enable them to retain their purity； and as soon as they are left to themselves they begin to deteriorate through accidental crosses and admixtures and rapidly return to the mixed condition。
Reverting now to Darwin's discussion of the variability of cereals； we may conclude that subsequent investigation has proved it to be exactly of the kind which he describes。 The only difference is that in reality it reaches a degree； quite unexpected by Darwin and his contemporaries。 But it is polymorphic variability in the strictest sense of the word。 How the single constituents of a variety originate we do not see。 We may assume； and there can hardly be a doubt about the truth of the assumption； that a new character； once produced； will slowly but surely be combined through accidental crosses with a large number of previously existing types； and so will tend to double the number of the constituents of the variety。 But whether it first appears suddenly or whether it is only slowly evolved we cannot determine。 It would； of course； be impossible to observe either process in such a mixture。 Only cultures of pure races； of single…parent races as we have called them； can afford an opportunity for this kind of observation。 In the fields of Svalof new and unexpected qualities have recently been seen； from time to time； to appear suddenly。 These characters are as distinct as the older ones and appear to be constant from the moment of their origin。
Darwin has repeatedly insisted that man does not cause variability。 He simply selects the variations given to him by the hand of nature。 He may repeat this process in order to accumulate different new characters in the same family； thus producing varieties of a higher order。 This process of accumulation would； if continued for a longer time； lead to the augmentation of the slight differences characteristic of varieties into the greater differences characteristic of species and genera。 It is in this way that horticultural and agricultural experience contribute to the problem of the conversion of varieties into species； and to the explanation of the admirable adaptations of each organism to its complex conditions of life。 In the long run new forms； distinguished from their allies by quite a number of new characters； would； by the extermination of the older intermediates； become distinct species。
Thus we see that the theory of the origin of species by means of natural selection is quite independent of the question； how the variations to be selected arise。 They may arise slowly； from simple fluctuations； or suddenly； by mutations； in both cases natural selection will take hold of them； will multiply them if they are beneficial； and in the course of time accumulate them； so as to produce that great diversity of organic life； which we so highly admire。
Darwin has left the decision of this difficult and obviously subordinate point to his followers。 But in his Pangenesis hypothesis he has given us the clue for a close study and ultimate elucidation of the subject under discussion。
V。 HEREDITY AND VARIATION IN MODERN LIGHTS。
By W。 BATESON； M。A。； F。R。S。
Professor of Biology in the University of Cambridge。
Darwin's work has the property of greatness in that it may be admired from more aspects than one。 For some the perception of the principle of Natural Selection stands out as his most wonderful achievement to which all the rest is subordinate。 Others； among whom I would range myself； look up to him rather as the first who plainly distinguished； collected； and comprehensively studied that new class of evidence from which hereafter a true understanding of the process of Evolution may be developed。 We each prefer our own standpoint of admiration； but I think that it will be in their wider aspect that his labours will most command the veneration of posterity。
A treatise written to advance knowledge may be read in two moods。 The reader may keep his mind passive； willing merely to receive the impress of the writer's thought； or he may read with his attention strained and alert； asking at every instant how the new knowledge can be used in a further advance； watching continually for fresh footholds by which to climb higher still。 Of Shelley it has been said that he was a poet for poets： so Darwin was a naturalist for naturalists。 It is when his writings are used in the critical and more exacting spirit with which we test the outfit for our own enterprise that we learn their full value and strength。 Whether we glance back and compare his performance with the efforts of his predecessors； or look forward along the course which modern research is disclosing； we shall honour most in him not the rounded merit of finite accomplishment； but the