THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
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that are introduced by the other sources of variance. For instance, if one-third
of the variance in milk-yield of a breed of dairy cows is genetic in this narrow sense of
the word, and if the breeders pay no attention to bulls, but select the cows so
intensively that those saved for breeding average 150 gallons more than their
entire contemporary generation, one can expect the next generation to average
25 gallons above the generation in which their parents were born. When
compared with the 150-gallon ' selection differential', the 25-gallon gain may
appear disappointingly small. No doubt this explains the fact that in every
country the voices of zealous enthusiasts are sometimes heard proclaiming that
the old methods of selection and attention to pedigree are useless, but that the
latest panacea of progeny test, sire index, or what not, will accomplish all that the
old methods are condemned for not doing.
In the example just quoted, the complete and exhaustive use of a progeny
test for the selection of bulls, while still using the same standard of selection for the
cows, could about double the rate of progress, the exact figure depending much upon
what portion of the tested sires it is finally necessary to save for replacement pur-
poses, and on what portion of the dams of the future breed are sired by bulls being
tested, and what portion can be sired by bulls after they have been proven. Such
a doubling of the rate of progress would be an achievement important enough to
justify much enthusiasm for proven sires. Yet the point I wish to make here is
that the difference is not between methods that are wholly right and methods that
are wholly useless, but between methods of selection that use more or use less of the
information which is available or might be made available for selecting. The
difference is in degree more than in kind. Too often the old method is condemned
because it does only what a more careful study and measurement would lead us to
expect it to do, whilst a method heralded as new is, on account of its plausibility,
extolled far beyond what we really have a right to expect of it.
For further example : if the selection practised were only that of saving bulls
from the high-producing cows, what has one a right to expect ? That is tanta-
mount to predicting cows' yields from the yields of their paternal grandams, and one
would expect such cows to exceed the average of what their generation would
have yielded without selection, by an amount approximately equal to one-fourth of
that fraction of the variance which is additively genetic, multiplied by the amount
by which the paternal grandams' yield actually exceeded the average of their
generation. Thus, with one-third of the variance genetic and a selection differen-
tial of 150 gallons for the paternal grandams one would expect an increase of only
about 12½ gallons in the grand-daughters. This might naturally seem almost
no increase at all to those who had been led by innuendo to think that they had a
right to expect in the grand-daughter an increase almost equal to the selection
differential practised on the grandams.
These calculations are crude and sketchy and do not include, for example,
what may be expected when selection is practised simultaneously on dams and sires,
grandams and grandsires, but naturally with varying intensities on each, and with