550 



BREEDING 



BREEDING 



crossing of different strains. By this method, he found that the 

 progeny of each individual fluctuated around an a/erage or typical 

 size, as had the commercial varieties, but that while some strains 

 were exactly the same in average size as the commercial variety, 

 others fluctuated around a larger mean or a smaller mean than the 

 commercial variety. He tried the experiment of selecting from 

 these large and small strains extreme variants, and found that no 

 advance was made as a result of the selection. He was thus led 

 to conclude that in a pure self -fertilized strain from a single plant 

 what he called a pure line no advance could be made by selection 

 and that the commercial variety with which he first experimented 

 was a mixed race. In the course of his experiments with pure lines, 

 several variations were obtained which reproduced true to type, 

 but these were interpreted by him as changes of type by mutation. 

 While, before the publication of Johannsen's results, breeders 

 clearly recognized the importance of determining individual per- 

 formance and using pedigree methods, still his pure-line conception 

 was a distinct advance and forcibly brought to attention the fact 

 that most commercial varieties and races consist of a number of 

 distinct types biotypes, as he ealled them, and that much of 

 our work of selection consists merely in isolating and purifying 

 these types. 



Is one, then, to conclude that the practice of breeders 

 in continually selecting from the best for propagation 

 is useless, and must one advise practical breeders to 

 discontinue their selection? There can be no doubt 

 that the practical breeders have made advances by 

 selecting from the best individuals. No scientific 

 breeder will deny this. It is simply the question of the 

 interpretation of how the results were secured that is 

 in doubt and whether these results can be considered 

 as permanent new unit^characters. 



It appears that one is dealing in breeding with two 

 markedly distinct types of selection, based on different 

 principles and arriving at different results, both correct 

 in principle and productive of equally valuable practical 

 results, but of very different value when considered 

 from a strictly evolutionary standpoint. The first of 

 these types would be that in which mutations are 

 selected and new races established, while the second 

 would be illustrated by that type of selection which is 

 intended merely to maintain a maximum strain of the 

 race. 



It would seem that such cases of improvement as 

 are illustrated by the sugar-beet indicate that the con- 

 tinuous selection, generation after generation, of maxi- 

 mum fluctuations shown by a character, will result in 

 maintaining a strain at nearly the maximum of effi- 

 ciency; and that within a pure race the progeny of a 

 maximum yariate which would probably be classed 

 as a fluctuation, does not regress entirely to the mean of 

 the race in the first generation succeeding the selec- 

 tion, but that there is only a certain percentage of re- 

 gression similar to the regression determined by Galton. 



These races or selected strains maintain themselves 

 as long as the selection is continued, and when the 

 selection is discontinued rapidly regress to the mean of 

 the species. 



The practical breeder should clearly recognize that 

 the act of selection, the choice of the best, remains just 

 as important whether it has a cumulative effect, 

 thereby augmenting the character, or whether he is 

 merely purifying an already existing superior race. The 

 final result remains the same. 



Methods of selection, or pedigree breeding. 



By methods of selection is meant those practices that 

 the breeder uses to find promising variations, determine 

 their value, and purify or develop them into fixed 

 races coming true to seed. 



Choosing superior plants. 



The first concern of the breeder is to find the valuable 

 variations. How he had best do this will depend largely 

 upon the plant with which he is working. In all cases, 

 it is of the greatest importance to find the best possible 

 plants and this is likely to require the examination of a 

 very large number of individuals. This factor cannot 

 be too strongly emphasized. If, for example, one at- 

 tempted to find a man 7 feet high, one would probably 



have to examine, or pass over, a million individuals 

 to find him. The superior individuals fitted to be the 

 progenitors of a new or improved race are very few. 

 Certain individuals far above the average may be 

 found by examining a comparatively limited number, 

 but the very best possible individual is but rarely 

 produced. 



The plants from which selections are to be made 

 should be grown under as uniform conditions as possible, 

 so that the experimenter may have opportunity to 

 examine and select the best. Two methods of growing 

 plants for selection are in general use, and may be 

 termed the nursery method and the field method. 



The nursery method, which was first used by Hallett 

 about 1868, consists in cultivating each plant under 

 the most favorable conditions possible for its best 

 development. By this method with wheat, for example, 

 Hallett pursued the policy of planting the indidivuals 

 in squares a foot apart, which would give each plant 

 abundant opportunity for stooling, and also the investi- 

 gator an opportunity clearly to distinguish each indi- 

 vidual plant and determine its characteristics, total 

 yield, and so on. In recent years, this method of grow- 

 ing the individual plants at a standard distance from 

 each other, in order to test their yielding capacities and 

 the like, has been used very extensively. 



The field method was used by Rimpau about 1867, 

 and probably by many others before that time. By 

 this method, the selections are made from plants grown 

 under normal field conditions. The advantages of this 

 method are that it can be judged only what a plant will 

 do in the field under ordinary conditions of field cul- 

 ture, by growing and selecting it under these conditions. 

 In the large majority of cases, the first selections are 

 probably made from plants grown in the field in the 

 regular course of crop-production, which thus were 

 not specially grown for the purpose. 



If one is to use the nursery method, the plants must 

 be especially planted. While the nursery method cer- 

 tainly allows the breeder to distinguish the individual 

 plants more clearly, in wheat, oats, and other crops that 

 are sown broadcast or drilled, it entails very much 

 extra work and is probably to be recommended only for 

 the use of experimenters who are giving their entire 

 time to the work. In the greater number of horticultural 

 crops, the individuals are normally cultivated one in a 

 place, as in the case of tomatoes, cabbages, strawberries, 

 currants and the like, and the examination of individuals 

 in the field thus satisfies the requirements of both 

 the above methods. 



The breeder may have in mind either of two pur- 

 poses in his work: (1) On the one hand, be may desire 

 to secure an improved strain of a certain race, that is, 

 by selection to keep his seed up to the maximum of 

 efficiency. This may be called strain breeding. (2) On 

 the other hand, he may desire to produce an entirely 

 new race with different characters, and this may be 

 called race-breeding. 



He should clearly recognize which of these types of 

 breeding he is following. As an illustration, suppose 

 that the breeder is growing the Stone tomato and 

 desires to maintain the best-yielding strain possible of 

 this race. He would then attempt to choose from a very 

 large number of plants of the Stone variety, the best- 

 yielding plants having the largest number of perfect 

 fruits and typical of the variety in habit of growth, 

 quality, character of fruit, and the like, and would hope 

 by a process of continuous selection to maintain his 

 selected strain in a state of high productivity. This is 

 the type of selection pursued by the sugar-beet 

 breeders described earlier in this article. 



On the other hand, if he desires to produce an im- 

 proved new race, he would search among large numbers 

 of tomato plants of any or all varieties for the appear- 

 ance of mutations or sports, or plants of new type differ- 

 ing from any known variety. As a matter of experience, 



