GENERAL ZOOLOGY 



Galton on human height has been supplemented by investigations of inherit- 

 ance of eye color, mental ability, length of life, and other characteristics by 

 Karl Pearson, Raymond Pearl, and other biometricians. Under circum- 

 stances where experimental analysis and study of individual pedigrees are 

 impossible, as is the case for most human characteristics, the biometrical 

 method can determine trends and reveal suggestive correlations. The results 

 of such studies do not make possible predictions concerning the inheritance 

 of traits by the progeny of specific crosses. 



Galton's observations, although made on groups that were not subjected to 

 experimental control, furnished the stimulus for the experimental work of 

 Wilhelm Johannsen, a Danish botanist (1857 1927). Johannsen conceived 

 the idea that if off^spring of parents who were above or below the average were 

 also above or below the average, respectively, as Galton had found, it might 

 be possible to shift the average by continued selection of parents from the 

 unusual groups. Working with beans in which self-fertilization occurs, he 

 chose the heaviest seeds from which to raise a new generation. The seeds 

 selected weighed 0.8 gram each, and from them Johannsen obtained plants 

 that produced seeds varying in weight from an average of 0.35 gram on some 

 plants to an average of 0.6 gram on others. He next tried similar experiments 

 with the seeds from single plants. The selection of the smallest seed or the 

 largest seed for planting had no eflfect on the size range of the seeds produced. 

 It was not possible to grow larger and larger beans by planting the largest 

 seeds time after time. Since cross-fertilization did not occur, the fundamental 

 hereditary constitution was not altered throughout the experiment. Beans 

 descended by self-fertilization from any single plant constitute what is known 

 as a pure line. Within pure lines Johannsen found that, although environ- 

 mental factors of temperature, moisture, or soil might affect the size range, 

 the average weight could not be shifted by selecting either the lightest or the 

 heaviest beans for planting. 



Johannsen was also able to demonstrate that in large groups of bean plants 

 many pure lines are represented. Such large groups of individuals are known 

 as populations. Just as each pure line varies around its average, the popula- 

 tion varies around the average of its component pure lines. Starting with a 

 population, it is possible to sort out, by selection of parents, lines having 

 desired characteristics. The practical breeder tends to do this in his selec- 

 tions of breeding stock and seeds. However, pure lines are stable only if 

 cross-breeding is prevented, which is frequently impossible. Selection within 

 pure lines has been tested by many investigators. Experiments have been 

 conducted on inheritance of size variation in different organisms, chemical 

 content of potatoes and sugar beets, egg-laying capacity in poultry, time of 

 maturity of seeds, distribution of color in coats of mammals, and numerous 

 other characteristics. In no case has selection shifted the average about 

 which a pure line varies. Such variations, which are known as fluctuations, 

 are conditioned by environmental effects during development and do not in- 

 fluence the characteristics of succeeding generations. 



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