een able to
collect, rendering it probable that climate, food, &c., have acted so
definitely and powerfully on the organisation of our domesticated
productions, that they have sufficed to form new sub-varieties or races,
without the aid of selection by man or of natural selection. I will then
give the facts and considerations opposed to this conclusion, and finally
we will weigh, as fairly as we can, the evidence on both sides.

When we reflect that distinct races of almost all our domesticated animals
exist in each kingdom of Europe, and formerly even in each district of
England, we are at first strongly inclined to attribute their origin to the
definite action of the physical conditions of each country; and this has
been the conclusion of many authors. But we should bear in mind that man
annually has to choose which animals shall be preserved for breeding, and
which shall be slaughtered. We have also seen that both methodical and
unconscious selection were formerly practised, and are now occasionally
practised by the most barbarous races, to a much greater extent than might
have been anticipated. Hence it is very difficult to judge how far the
difference in conditions between, for instance, the several districts in
England, could have sufficed without the aid of selection to modify the
breeds which have been reared in each. It may be argued that, as numerous
wild animals and plants have ranged during many ages throughout Great
Britain, and still retain the same character, the difference in conditions
between the several districts could not have modified in so marked a manner
the various native races of cattle, sheep, pigs, and horses. The same
difficulty of distinguishing between selection and the definite effects of
the conditions of life, is encountered in a still higher degree when we
compare closely allied natural {273} forms, inhabiting two countries, such
as North America and Europe, which do not differ greatly in climate, nature
of soil, &c., for in this case natural selection will inevitably and
rigorously have acted during a long succession of ages.

From the importance of the difficulty just alluded to, it will be
advisable to give as large a body of facts as possible, showing that
extremely slight differences in treatment, either in different parts of
the same country, or during different seasons, certainly cause an
appreciable effect, at least on varieties which are already in an
un