BREEDING 



BREEDING 



549 



practical breeding has not been determined, and indeed further 

 experience must be awaited before the evidence, or the interpreta- 

 tion of the evidence presented in these very valuable and sug- 

 geative researches, can finally be accepted. Humbert has made ex- 

 periments in which the capsules of a pure line of a wild plant 

 {Silene noctijlora) were injected with the solutions used by 

 MacDougal, and although the number of plants handled (about 

 15,000) w-as apparently as great or greater than was used in Mac- 

 Dougal's experiments, no mutations were found in the treated 

 plants that were not also found in the untreated or check plants. 



Some observations and experiments are recorded in literature 

 which indicate that mutilations or severe injury may induce the 

 development of mutations. Most noteworthy among such obser- 

 vations are those of Blaringhem, who by mutilating corn plants in 

 various ways, such as splitting or twisting the stalks, apparently 



Eroduced variations that bred true without regression and which 

 e described as mutations. Observations on the great frequency of 

 striking bud-variations on recovering trunks of old citrus trees in 

 Florida, following the severe freeze of 1894-5, also furnished 

 evidence in support of this theory. 



While the e\'idence at command regarding the artificial pro- 

 duction of mutations is not yet sufficiently exact and trustworthy 

 to enable one to draw definite conclusions and formulate recom- 

 mendations for practical breeders, it may be stated that this is 

 apparently one of the most profitable lines of experimentation for 

 the immediate fiituTf 



Principles of selection. 



Selection is the principal factor of breeding, both in 

 the improvement of races and in the production of 

 new races and varieties (Fig. 641). The keynote of 

 selection is the choice of the best, and a factor of the 

 highest importance in finding the best is the examination 

 of very large numbers. 



In evolutionary studies, it has long been recognized 

 that variation is the foundation of evolution and that 

 no evolution is possible without variation; but, to selec- 

 tion has been assigned an all-important part as guiding 

 and even stimulating the variation in a certain direc- 

 tion. Darwin, and particularly some of his more radical 

 followers, have assigned to selection a creative force, in 

 that it has been assumed that when nature by a slight 

 variation gave the hint of a possible change in a certain 

 direction, natural or artificial selection, by choosing this 

 variation and selecting from among its progeny the 

 most markedly similar variants, could force the advance 

 in the direction indicated. Since Darwin's time, this 

 cumulative action of selection had been emphasized so 

 forcibly that selection had come to be recognized as an 

 active force in creation rather than merely as a deter- 

 minative agency. 



It is certain, of course, that, bj' careful observation 

 and selection from any particular race, ultimately a 

 new race may be produced. The question is whether 

 the individual or individuals selected in producing the 

 new race have not varied by mutation or seed-sporting 

 rather than being merely representative of the cumula- 

 tive result of the selection of slight individual varia- 

 tions. The sugar-beet furnishes an interesting illustra- 

 tion in this direction. It will be remembered that 

 Louis Vilmoiin started the selection of sugar-beets for 

 richness in sugar between 1830 and 1840, selecting first 

 by means of specific gravity, the method being to throw 

 the beets into solutions of brine strong enough so that 

 the great majority of them would float, the few that 

 sank being of greater specific gravity and presumably 

 of greater sugar-content. Considerable improvement 

 was produced by this method. About 1851, the method 

 of chemical analysis was introduced to determine the 

 exact sugar-content. At this time, the sugar-content 

 was found to var>- from 7 to 14 per cent, and in the 

 second generation of selection individuals with 21 per 

 cent of sugar were found. The selection based on per- 

 centage of sugar, using the beets highest in sugar as 

 mothers, has been continued regularly since that time, 

 and the industry has come to rely entirely on careful 

 selection for high sugar-content. It would be expected 

 that under these conditions, the percentage of sugar 

 would have increased sufficiently so that the selected 

 plants could be considered a different race orstrain. Yet, 

 after fifty years of selection, the highest sugar-content 

 found is only about 26 per cent, and this in a very few 



641. Improvement of corn by 

 selection. Boone County White 

 corn on left, and original type 

 from which it was developed by 

 selection on right. 



instances, seldom over 21 per cent being found. At the 

 present time, many thousand analyses are made every 

 year, so that abundant opportunity is afforded to find 

 individuals producing a high sugar-content. On the 

 contrary, when Vilmorin's 

 work was started, the 

 determination of sugar- 

 content was made by 

 very laborious methods, 

 and was limited to com- 

 paratively few individ- 

 uals. It is not improb- 

 able that if Vilmorin had 

 been able to make analy- 

 ses of the sugar-content 

 in many thousands of 

 roots, he would have 

 found certain individuals 

 producing as high as 26 

 per cent. The inference 

 from this illustration 

 would be that the limita- 

 tions of the variation 

 within the race have not 

 been surpassed as a result 

 of selection. 



Of recent studies favoring the active influence of selection in 

 creating or strengthening characters, the most noteworthy are those 

 of Castle and Smith. 



Castle and his assistants made an extensive series of experiments 

 with hooded rats to increase the black-colored dorsal band on the 

 one hand and to decrease or obliterate it, on the other. He appears 

 to have obtained very positive evidence favoring the gradual cumu- 

 lative action of the selection, as he succeeded in markedly increasing 

 the amount of black in one strain until the rats were almost wholly 

 black and in the other strain almost wholly obliterating the black. 

 Castle has also obtained similar results in producing a four-toed 

 race, and a change of coloring in guinea-pigs. His view may be 

 summarized in the following quotation; "In Johannsen's view, 

 selection can do nothing but sort out variations already existing in 

 a race. I prefer to think with Darwin that selection can do more 

 than this; that it can heap up quantitative variations until they 

 reach a sum total otherwise unattainable, and that it thus becomes 

 creative." 



The experiments conducted by Smith and others at the 

 Illinois Experiment Station on selecting high and low strains of 

 corn with reference to oil- and protein-content, have resulted in 

 markedly distinct strains possessing these qualities. Experiments 

 have also been made in cultivating these varieties without selection 

 and the new characters have been maintained for several years 

 without marked regression. 



The standard researches of DeVries, now familiar to all, chal- 

 lenged the correctness of the selection theory and sought to show 

 that species originated by sudden jumps or mutations. It may be 

 admitted that DeVries proved that species or new characters 

 were formed suddenly as mutations, but this would not prove 

 that they might not also be formed or actually induced to mutate 

 by a continuous process of selection. Indeed, in his experiments on 

 the production of a double-flowered variety of Chrysanthemum 

 segetum ("Mutationstheorie," Vol. I, p. 523), a few generations of 

 selection led to increasing markedly the number of ray-florets 

 before the ligulate corollas appeared among the disk-florets, the 

 change that he interpreted as the mutation that gave him the double 

 variety. 



Tower's experiments with the potato beetle in attempting to 

 create by selection large and small races, albinic and melanic races, 

 and races with changed color-pattern, although conducted carefully 

 from ten to twelve generations, failed to give any evidence of pro- 

 ducing permanently changed types. While strains of plus and 

 minus varieties gave populations with a range of variation appar- 

 ently markedly re-^tricted to their respective sides of the normal 

 variation range, still these selected strains did not greatly exceed 

 the normal range of variation in either direction, and when the 

 selection was discontinued, in two or three generations, again 

 populations exhibiting the normal range of variation were produced. 

 Jennings, in a series of selection experiments with Paramecium 

 extending over twenty generations, and Pearl, in an extensive 

 experiment in the selection of chickens in an attempt to produce a 

 breed of high egg-laying capacity, failed to secure any evidence 

 favoring an accumulative effect of selection. 



Xo series of experiments have had a more profound influence on 

 the conception of selpction than those of Johannsen, the Danish 

 investigator. In studying commercial varieties of beans, he found 

 that such characters as weight and size of seed fluctuated around a 

 certain average, and when large seed or small seed was chosen, the 

 progeny showed the influence of the selection, being smaller or 

 larger in accordance with the direction of the selection. The 

 progeny, however, did not exhibit the extreme sizes of the selected 

 parents, there being a certain regression toward mediocrity. In 

 investigating this matter, Johannsen was led to use the ordinary 

 pedigree method of cultivating the progeny of different individuals 

 separately and inbreeding or selflng all seed used to prevent the 



