ABSTRACTS
251
men have long believed this, but the greater concordance in tuberculosis infection in
identical twins than in fraternal twins gives solid backing to this belief. The weight of the
evidence is increased by cases in which the site of infection and other details are very
similar.
An example chosen from plants will show how the interrelation between nature and
nurture affects profit and loss. Certain native sugar-canes in India yielded 10 to 12
tons an acre. The plant breeders got busy and produced new canes which, with heavier
manuring, better regulated irrigation, and other superior care, gave up to 35 tons. The
' improved canes ' were useless in the original conditions, and under the higher
standard of management the old canes failed to ripen and gave very low sucrose content.
So closely interlinked are the efforts of plant breeder and plant husbandman. The
internal and external had to change in harmony.
For progress in animal or plant breeding the geneticist must cooperate with workers
in other branches of agricultural science. Intimate knowledge is needed—in this head
or in that-of structural details, developmental history, life habits, relation with environ-
ment, pathology, and different branches of physiology. The advancing of livestock pro-
duction is a task for a team.
Breeding methods in the light of genetics.—If too much of the restricted power of
choice for one particular character is used to the enforced automatic neglect of other
important traits, on balance more harm than good may result. It is probably a result
of this simple arithmetical principle that high egglaying strains of fowls have tended to
deteriorate in vigour.
A polled bull is either a homozygote or a heterozygote. If he is the former, you may
safely promise that all his calves will be polled. If he be the latter, when he is mated with
horned cows half his offspring, on the average will be polled, half horned, and any excess
of polled would be a chance deviation. To be sure, if a heterozygous bull were mated
with cows all heterozygous, three-quarters of the offspring would—barring chance varia-
tion in ratio—be the most he could do, and if some of his polled mates were homo¬
zygotes a larger proportion of the calves than three in four would be free from horns.
To invoke telegony is to offer one explanation of why progeny may be unlike either
parent, but nowadays we think readily in terms of the recombination of factors. A strong
reason for doubting the reality of telegony is that what we know of the mechanism of
heredity—of genes and chromosomes—makes it difficult to understand how the alleged
influence of an earlier sire on later progeny could be produced. That objection, however,
is not conclusive, for the mere fact that we do not at the moment understand how a thing
can happen does not mean that it cannot happen. A weightier reason is that in an
immense number of laboratory experiments carried out for other purposes matings
have been made which could reveal the results of telegony were it a real phenomenon.
It is sometimes believed that with the persistent use of a certain sire the latter offspring
tend to resemble the sire more than the earlier ones.
As an example of supposed infection of the male is the claim that a white ram mated
with black ewes is more likely than he otherwise would be to sire black lambs when he is
afterwards mated with white ewes. Placing reliance upon maternal impressions, breed-
ers have been known to paint their buildings and fences a certain colour in the hope
of producing that particular colour in the offspring. These beliefs may be discarded on
grounds similar to those for rejecting telegony.
The sire is more carefully chosen than the dams, precisely because one male can be
mated with a number of females. This explains why the sire may be more important
in determining characterization than the dams. We may note further that the sire may
appear to be the more important simply becuase he has a better chance to get a good
number of offspring strikingly alike than have the dams. This depends upon dominance
and homozygosity.
In race-horses and in cattle especial importance is sometimes attributed to descent
in the direct female line from some remote and famous ancestress. But in each genera-
tion half the genes come from the male, and chromosomes assort at random when the
germ cells are produced. After six or eight or ten generations low indeed is the propor
tion of genes in an individual likely to be derived from that-very-great-grandmother in
the direct female line, or from any other one of the crowd of ancestors that same number
of generations back.