Full text: Problems in eugenics

44Exhibit C 109—ni. 
C 110 
C 111an extensive use of it with the view to the cultivation of certain here­ 
ditary characteristics. 
We show in Table C 110, after de Chapeaurouge, the pedigree 
of Eelvidere, an animal which, in spite of close inbreeding, was 
distinguished by excellent qualities, and by whom, out of his own 
daughter, another sire of the highest rank was produced. 
After long-continued and very close inbreeding, even with a faultless 
condition of the germ plasm, the decrease of vitality and fertility 
of the progeny asserts itself. Important evidence for this is given by 
Georg. H. Shull in his exhibition of cross-fertilized, self-fertilized 
and hybridized maize (Exhibit No. C 111). Shull makes the 
following comments : <! Results of inbreeding with maize—crossing 
between different races or genotypes, if not too distantly related, 
results in a progeny which excels its parents in vitality, whereas 
crosses between individuals belonging to the same genotype engender 
no increase of vitality as compared with the parents.” 
In maize, and presumably in most other plants and animals 
in which cross-fertilization is the rule, all individuals are usually 
complicated hybrids between different varieties of genotype. They 
owe their vigorous constitution to this hybrid nature. 
“ The result of self-fertilization or of close inbreeding is that 
the hybrid nature diminishes in degree. The stock is reduced to a 
homozygotic condition, and is thus deprived of the stimulus which 
lies in the hybrid condition.” 
“ When two given genotypes are crossed, the first hybrid genera­ 
tion is possessed of the greatest vigour. Even the second generation 
shows much less vitality, and this decrease continues with the third 
and later generations. But each succeeding generation differs less 
from its predecessor than the latter differed from its own parents. 
As soon as the stock has become a pure line, inbreeding produces no 
further weakening. ’ ’ 
“ The top row of the exhibited collection of maize cobs (large 
cobs with many grains) is derived from a family in which for five 
generations self-fertilization has been prevented by using mixed 
pollen. These conditions approach those prevailing in an ordinary 
field.” “ The middle row of maize cobs (small cobs with few grains) 
comes from families of the same derivation as the first row; but for 
five generations they have been self-fertilized. Each one has char­ 
acters which the others do not possess. They are almost pure bred, 
and continued self-fertilization produces no further adverse influence. 
The cob, quite to the right, without grains, has pistils so short that
	        

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