550 



BREEDING 



BREEDING 



crossing ot differpnl strains. Hy this method, he found that the 

 prvitctmy ot" each indiviiiual tltictviated around an a.'oraKe or typical 

 siic. as had the commercial varieties, but that while some sti^ains 

 mT\" exactly the same in average sire as the commercial variety, 

 others Huctuated around a hirm^r mean or a smaMer mean than the 

 commerciid variety. lie tried the experiment of selecting from 

 lhei«e large .and small stniins extreme variants, and found that uo 

 ■dvanc*' wa^ made -a;* a result of the selection. He was thus led 

 to conclude that in a puri- 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 wliich he first experimented 

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

 several variations were obtained which reproduced true to type, 

 but thes*- were interpreted by him as changes of type by iimtation. 

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

 clearly rt*cognized the importance of determining indi\'idual per- 

 formance and using pctiigri'e methotls. still his pure-line conception 

 was a distinct advance and forcibly brought to attention the fact 

 that most commercial varieties anil races consist of a number of 

 distinct types — biotypes. as he called them, — and that much of 

 our work of selection consists merely in isolating and purifying 

 the** liTx?^. 



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

 in continually selecting from the best for propagation 

 is useless, and must one advi.se practical breeders to 

 discontinue their selection? There can be no doubt 

 that the practical breeders ha\-e 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 

 marktxlly 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 variate 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 piarifying an already existing suj)erior race. The 

 final result remains the same. 



Methods of selection, or pedigree breeding. 



Bv met h'xls of selection Ls meant those practices that 

 the r)re<^Jer u-ses to find promising variations, determine 

 their valije, and purify or develop them into fixed 

 races coming true to seed. 



Chooxing nuperir/r pltmts. 



The first concern of the breeder is to find the valuable 

 variation-s. How he had best do this will depend largely 

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

 it Ls of the greatr-st imfX)rtance to find the best po.ssible 

 plants and this is likely trj require the examination of a 

 very large number of individuals. This factf)r cannot 

 be U)t> strongly empha-sized. 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 imjiroved race are very few. 

 Certain indi\-iduals far above the average may be 

 found by examining a coniparativelj- limited number, 

 but the very best possible individual is but rarely 

 produced. 



The plants from which selections are to be made 

 should l)e grown under as uniform conditions as possible, 

 so that the exiierimcnter may have opportunity to 

 examine and select the best. Two methods of growing 

 plants for selection are in general use, and maj' be 

 termed the nurser>' method and the field method. 



Tlw nursenj method, which was first used bj' Hallett 

 about 1S68, 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 jilants 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 ver}- 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, he 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 hantl, 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, 



