the one at the expense of
the other until the milking quality in many families is nearly bred
out. It was not so formerly--thirty years ago the Short-horns (or as
they were then usually called, the Durhams) were not deficient in
dairy qualities, and some families were famous for large yield. By
properly directed efforts they might, doubtless, be bred back to milk,
but of this there is no probability, at least in England, for the
tendency of modern practice is very strong toward having each breed
specially fitted to its use--the dairy breeds for milk and the beef
breeds for meat only. The requirements of the English breeder are in
some respects quite unlike those of New England farmers--for instance,
as they employ no oxen for labor there is no inducement to cultivate
working qualities even, in connection with beef.

As an illustration of the effect of habit, Darwin[5] cites the
domestic duck, of which he says, "I find that the bones of the wing
weigh less, and the bones of the leg more, in proportion to the whole
skeleton, than do the same bones in the wild duck; and I presume that
this change may be safely attributed to the domestic duck flying much
less and walking more than its wild parent." And again, "not a single
domestic animal can be named which has not in some country drooping
ears, and the view suggested by some authors, that the drooping is due
to the disuse of the muscles of the ear, from the animals not being
much alarmed by danger, seems probable."

Climate, food and habit are the principal causes of variation which
are known to be in any marked degree under the control of man; and the
effect of these is, doubtless, in some measure indirect and
subservient to other laws, of reproduction, growth and inheritance, of
which we have at present very imperfect knowledge. This is shown by
the fact that the young of the same litter sometimes differ
considerably from each other, though both the young and their parents
have apparently been exposed to exactly the same conditions of life;
for had the action of these conditions been specific or direct and
independent of other laws, if any of the young had varied, the whole
would probably have varied in the same manner.

Numberless hypotheses have been started to account for variation. Some
hold that it is as much the function of the reproductive system to
produce individual differences as it is to make the child like the
parents. Darwin says "the reproductive system