THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK
273
this, for example, may be illustrated by the following average fat-measurements
of the loins (Fig. 6, Plate XVII) of carcasses at Smithfield Show (Fig. 7—by
Hirzel, p. 275). First crosses are intermediate between parents in this respect,
and there is no dominance or recessiveness of such developmental characters.
On the other hand, an alteration of the proportions of the body brought about
by a mutation (and not of a developmental character), such as that of the short
legs of the Ancon sheep (Fig. 8, Plate XVIII), behaves as a recessive segregating
character, and does not blend with all gradations as do the developmental charac-
ters. The developmental characters must always be represented by a variability
curve, and never as a fixed point, as the mutations can be. Improvement of the
environment and selection of the animals at the upper end of the curve, for the
curve shifts upwards as the result of environmental changes, are the best means
of effecting improvements for early maturity.
Pigs (bacon, pork and lard).—Local feed conditions have supplied the environ-
ment in which the different types of pigs have been evolved. For example, in the
maize-producing areas of Hungary and Roumania, the Mangalicza pig has been
developed for fat-production ; this pig has only a small development of bone and
muscle, but the back fat, which it is bred for, reaches, in a good average pig of
390 lb. live-weight, a thickness of 5 in. at the shoulder and 4 in. at the loin. In
the corn-belt of America, too, the Poland-China breed developed in the same
way (Fig. 9, Plate XVIII) but the type within the breed has been changed in recent
years (owing to the lack of demand for lard) by selection, and the much greater
use of proteins in the ration. Under the feeding conditions existing in Denmark
(skim milk and cereals), the bacon pig (in which a carcass of 150 lb., thick in lean,
and with only a moderate amount of fat, is required) has reached its highest
development. Progeny test of growth and carcass quality, made under feeding
conditions that stimulate the characters required, are the means whereby this has
been achieved. It has been a directed evolution of commercial qualities by the
accumulation of small increases (as the progeny tests of their boars show), and not
by the appearance of sudden large mutations. For pork (where a large development
of the muscle at a low carcass weight—70 lbs.— is required), it is in environments
supplying a ration high in protein that the highest perfection is attained. New
Zealand, for example, with its large supply of skim milk and meat meal is produc-
ing this type of pig to perfection from breeds as diverse as the Berkshire, Large
White, and Tamworth (see Fig. 10, Plate XIX). The high-protein and low-carbo-
hydrate feed leads to the development of the muscle and limits the fat to the pro.
portions required by the consumer, whereas under our conditions of high-carbo-
hydrate feeding the differences in fatness between these breeds would be very
marked. In general, the degree of fixity of the commercial character will depend
on the order of its development in the animal; thus bone, which develops first, is
more difficult to modify the local feeding condition than is fat, which