THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
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which different individuals have, whilst faintly hereditary characteristics are those
in which most of the variance ordinarily occurring is due to differences in environ-
ment to which different individuals have been exposed. Characteristics may
further be classified according to whether the hereditary portion of the variance
is due to few factors and shows sharply discontinuous classification, or many
genes are involved and yield a practically continuous series of phenotypes. From
this standpoint the differences between highly hereditary and slightly hereditary,
or between continuous and discontinuous, characteristics, although highly im-
portant practically, involve no differences in fundamental principles, but only
differences in the number of genes involved, or in the proportions of the total
observed variance resulting from genetic differences and from environmental
differences.
So much for the fundamental facts concerning the inheritance of productivity.
Before applying those facts to breed higher productivity into a breed or strain of
animals, as an engineer would apply the fundamental principles of physics and
mechanics to such a task as the building of a bridge, measurements are needed of
the magnitude of the forces involved, just as the engineer would need measure-
ments of the loads his bridge was expected to carry, the amount and
seasonal distribution of the water-flow which must move down the stream under it,
the number of years before it is expected to become obsolete, the amount of money
available for construction, etc. I venture the opinion that, in general, this is the
least explored field of applied genetics, and that neglect to measure and integrate
properly these variables is the major cause for the mistakes and unsound proposals
that are sometimes made even by those whose knowledge of biological fundamen-
tals is not seriously deficient.
It will not have escaped notice that all four speakers have mentioned selection
as a first step in breeding for productivity, although they differ considerably in
their optimism as to its sufficiency. I would here call attention to only two aspects
of the question of how intense selection can be, even in the idealized case where
the man doing the selecting is possessed of complete knowledge of the genotypes
of his animals. The first is that natural fertility and longevity set serious limits
to the intensity of the selection which may be practised. Each parent must even-
tually be replaced by another, even in a population static in size. What propor-
tion of the female offspring must be saved for replacements ? What proportion of
the males ? Here is a whole field of what we might call' the vital statistics of farm
animals which is as yet only slightly explored. In dairy cattle, about 50 to 60
per cent of the heifers must be saved, merely to replace their dams, which are
being lost by disability, old age, and other causes of death. In swine, perhaps
10 per cent of the gilts would be enough ; perhaps 7 or 8 per cent would be enough
for exceptionally well-managed herds. In chickens, perhaps 2 per cent of the eggs
that might possibly be hatched into pullets would be enough for replacements.
These figures set serious and varying limits to intensity of selection in the first
place. One must begin with the material available. There is small use to talk