250
ABSTRACTS
whore the covering was left than on the side shorn in the ordinary way. This suggests
a method of camouflaging the genetic make-up by a device to which it may perhaps be
deemed not in the public interest to draw attention. The present point is that this is
one out of a number of lines of evidence showing that degree of hairiness depends on the
interplay of genetic and non-genetic forces. For fertility in sheep the same story can be
told. Investigators in a number of countries, including Lamont in New Zealand, have
shown that heredity counts, but the practice of flushing is a testimony to nutrition.
Indeed, flushing is sometimes purposely avoided in a highly fertile breed in order to
reduce the danger of unwanted triplets.
Environmental conditions operating before birth are also important. Statistical
work (Chapman and Hall) has shown that variance in the birth weight of lambs, a part
from the reduction of weight due to twinning, is largely brought about by non-genetic
factors. Recent research at Lincoln College (Leslie and McLean) gives us reason to
believe that the plane of nutrition before birth influences not only birth weight but
subsequent rate of gain, and also suggests that an improvement in the level of nutrition
about six weeks before birth slightly increases the abundance of halo-hairs, a feature
which the Massey College work has shown to be strongly hereditary.
In long-sustained work on pie-bald guinea-pigs, Wright has established a conclusion
on the relation between age of dam and amount of white in the offspring. As the age
of the mother increased from less than six months to more than twenty-one months the
amount of white in the young clearly increased. Beware, therefore, of attributing to
genetic factors too large a share of the variation in a colour pattern.
This list of examples has been given to show the need for distinguishing between the
effects of the internal and the external. In so far as we cannot do this, breeding progress
is checked, so that the disentangling of what comes from within and what from without
may well be accepted as a first-class problem in genetics. We will therefore list the lines
of attack which are appropriate, though these are obvious enough.
Firstly, control the breeding. Breed different stocks in the same environment, and
if the differences with which you start persist down the generations the conclusion is clear.
This is a starting project for the geneticist occupied with livestock, to find out simply
whether a character is inherited. He may not be able to identify individual factors, but
if he can show by mating like with like that genetic factors play an important or appre-
ciable part in determining whether pigs are long or short, and whether wool has lustre
or lacks it, that is useful and a beginning.
Sometimes you get surprises, selection being ineffective, there being no correlation
between characterization of parents and of offspring. From Wensleydale flock records,
which it was the author's privilege to analyse, it was shown that ewes of low fertility
produced daughters which themselves averaged as high a figure for fertility as the
daughters of more fertile ewes. High fertility is inherited, and so it is believed that the
whole flock was permeated with a factor or factors for high fertility, so that low produc-
tion of lambs by an individual ewe was a matter of chance.
Secondly, vary the environment. Use by preference stock as homogenous as
possible, closely inbred stock being favoured. Failing even stock, resort may be had to
large numbers. With due precautions the same animal may be treated differently at
different times. Or, as in differential shearing experiments, use different parts of the
same animal at the same time.
Nature sometimes makes us a present of handy experimental material in ' identical
twins', twins resembling one another with extreme closeness, which have developed
from a single fertilized egg, which by a split at an early stage of development has given
rise to two individuals identically endowed. Casual inspection of the contents of peram-
bulators, confirmed by statistics shows that identical twins are very common. The
two members of the pair are of the same sex. ' Fraternal twins ' differ as do brothers
and sisters born at different times, and may be of like or unlike sex. The careers of
identical twins reared apart have been followed, and the histories of identical and frater¬
nal twins have been compared. It has been shown clearly that both nature and nurture
are important in influencing both intellectual ability and temperament, and that an im-
portant role in the producton of crime must be ascribed to heredity. Studies of twins
have also afforded evidence that susceptibility to tuberculosis is inherited. Medical