o natural selection。 Relations to neighbouring plants and to injurious or useful animals； have been considered the most important determining factors ever since the time when Darwin pointed out their prevailing influence。
From this discussion of the main causes of variability we may derive the proposition that the study of every phenomenon in the field of heredity； of variability； and of the origin of new species will have to be considered from two standpoints； on one hand we have the internal causes； on the other the external ones。 Sometimes the first are more easily detected； in other cases the latter are more accessible to investigation。 But the complete elucidation of any phenomenon of life must always combine the study of the influence of internal with that of external causes。
III。 POLYMORPHIC VARIABILITY IN CEREALS。
One of the propositions of Darwin's theory of the struggle for life maintains that the largest amount of life can be supported on any area； by great diversification or divergence in the structure and constitution of its inhabitants。 Every meadow and every forest affords a proof of this thesis。 The numerical proportion of the different species of the flora is always changing according to external influences。 Thus； in a given meadow； some species will flower abundantly in one year and then almost disappear； until； after a series of years； circumstances allow them again to multiply rapidly。 Other species； which have taken their places； will then become rare。 It follows from this principle； that notwithstanding the constantly changing conditions； a suitable selection from the constituents of a meadow will ensure a continued high production。 But； although the principle is quite clear； artificial selection has； as yet； done very little towards reaching a really high standard。
The same holds good for cereals。 In ordinary circumstances a field will give a greater yield； if the crop grown consists of a number of sufficiently differing types。 Hence it happens that almost all older varieties of wheat are mixtures of more or less diverging forms。 In the same variety the numerical composition will vary from year to year； and in oats this may； in bad years； go so far as to destroy more than half of the harvest； the wind…oats （Avena fatua）； which scatter their grain to the winds as soon as it ripens； increasing so rapidly that they assume the dominant place。 A severe winter； a cold spring and other extreme conditions of life will destroy one form more completely than another； and it is evident that great changes in the numerical composition of the mixture may thus be brought about。
This mixed condition of the common varieties of cereals was well known to Darwin。 For him it constituted one of the many types of variability。 It is of that peculiar nature to which； in describing other groups； he applies the term polymorphy。 It does not imply that the single constituents of the varieties are at present really changing their characters。 On the other hand； it does not exclude the possibility of such changes。 It simply states that observation shows the existence of different forms； how these have originated is a question which it does not deal with。 In his well… known discussion of the variability of cereals； Darwin is mainly concerned with the question； whether under cultivation they have undergone great changes or only small ones。 The decision ultimately depends on the question； how many forms have originally been taken into cultivation。 Assuming five or six initial species； the variability must be assumed to have been very large； but on the assumption that there were between ten and fifteen types； the necessary range of variability is obviously much smaller。 But in regard to this point； we are of course entirely without historical data。
Few of the varieties of wheat show conspicuous differences； although their number is great。 If we compare the differentiating characters of the smaller types of cereals with those of ordinary wild species； even within the same genus or family； they 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 coun