THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK
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qualities, built up by small variations, constantly being added to according to the
environment of the animal, and the formation of varieties, freaks, and fancy points
produced by the mutation of genes already formed by the other process.
To illustrate the theories outlined above, I propose, in the short space avail-
able, to give a few examples. A concrete example may be given from the horse,
which although not among the meat-producing animals in Great Britain is closely
related to them. The evolution in the skeleton (Fig. 1, Plate XIV) has followed
a definite and uniform course of changes, consisting in the main of a progressive
lengthening of the limb bones in relation to cranium size. This evolution has not
been broken by a number of sharp changes, such as mutations affecting different
parts of the body independently ; for example, no shortened limb bones, such as
those which occur in the Dachshund dog (and also occasionally in the
horse) and behave as Mendelian recessives, come in the series. These
things are mutations that may easily be picked out by man and bred to
form fancy strains, but they play no real part either in natural evolution or
in the development of the proportions of the body in commercial meat-
production. These mutations do not, as a rule, form intermediates when bred to
the normal type, whereas when two different ' developed' types are crossed, all
gradations between them may be obtained. In the horse (see Plate XIV) the
different types of conformation are magnifications (light horses) or extensions
(heavy horses) of the gradual changes which have taken place during the course
of evolution.
Cattle (beef and veal).—Beef qualities, i.e., a high proportion of the best joints
(loin) and a low proportion of the offal parts (head and legs), are developmental
characters, and change in their proportions as the animal grows up (Fig. 2, Plate
XV). The head and legs are proportionally large in the calf, and as the beef quali-
ties develop they become proportionally smaller, and the loin becomes proportion-
ally larger. For the full expression of these developmental characters, a high
plane of nutrition is necessary, for, if it is not available, the later maturing and
more valuable parts are not developed and the form of the animal approaches
that of the unimproved type in which the head and legs are large and the loin
poorly developed (see Plate XV). If selection is made under poor conditions of
nutrition, therefore, we cannot distinguish so well between the one which is poor in
conformation due to lack of genetic improvement, and the one which is genetically
improved but fails to develop its body proportions because of lack of nutrition.
All the best breeds of beef cattle (Aberdeen-Angus, Shorthorn, Hereford) have been
developed in areas of good nutrition, and herds in poor nutrition (range) areas
become degenerate from a meat point of view, unless they are kept constantly
supplied with breeding-stock which has been selected in the areas of high nutrition.
When we consider the different breeds of cattle from the standpoint of the
development of body proportions for meat (Fig. 3, Plate XVI), it will be seen that
I