CHAPTER I
VARIATION UNDER DOMESTICATION
Circumstances favourable to Man's Power of Selection
I will now say a few words on the circumstances, favourable, or
the reverse, to man's power of selection. A high degree of variability
is obviously favourable, as freely giving the materials for
selection to work on; not that mere individual differences are not
amply sufficient, with extreme care, to allow of the accumulation of a
large amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept. Hence, number is of the
highest importance for success. On this principle Marshall formerly
remarked, with respect to the sheep of parts of Yorkshire, "as they
generally belong to poor people, and are mostly in small lots, they
never can be improved." On the other hand, nurserymen, from keeping
large stocks of the same plant, are generally far more successful than
amateurs in raising new and valuable varieties. A large number of
individuals of an animal or plant can be reared only where the
conditions for its propagation are favourable. When the individuals
are scanty, all will be allowed to breed, whatever their quality may
be, and this will effectually prevent selection. But probably the most
important element is that the animal or plant should be so highly
valued by man, that the closest attention is paid to even the
slightest deviations in its qualities or structure. Unless such
attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary
just when gardeners began to attend to this plant. No doubt the
strawberry had always varied since it was cultivated, but the
slightest varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or
better fruit, and raised seedlings from them, and again picked out the
best seedlings and bred from them, then (with some aid by crossing
distinct species) those many admirable varieties of the strawberry
were raised which have appeared during the last half-century.
With animals, facility in preventing crosses is an important element
in the formation of new races,- at least, in a country which is
already stocked with other races. In this respect enclosure of the
land plays a part. Wandering savages or the inhabitants of open plains
rarely possess more than one breed of the same species. Pigeons can be
mated for life, and this is a great convenience to the fancier, for
thus many races may be improved and kept true, though mingled in the
same aviary; and this circumstance must have largely favoured the
formation of new breeds. Pigeons, I may add, can be propagated in
great numbers and at a very quick rate, and inferior birds may be
freely rejected, as when killed they serve for food. On the other
hand, cats from their nocturnal rambling habits cannot be easily
matched, and, although so much valued by women and children, we rarely
see a distinct breed long kept up; such breeds as we do sometimes
see are almost always imported from some other country. Although I
do not doubt that some domestic animals vary less than others, yet the
rarity or absence of distinct breeds of the cat, the donkey,
peacock, goose, &c., may be attributed in main part to selection not
having been brought into play: in cats, from the difficulty in pairing
them; in donkeys, from only a few being kept by poor people, and
little attention paid to their breeding; for recently in certain parts
of Spain and of the United States this animal has been surprisingly
modified and improved by careful selection: in peacocks, from not
being very easily reared and a large stock not kept: in geese, from
being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of
distinct breeds; but the goose, under the conditions to which it is
exposed when domesticated seems to have a singularly inflexible
organisation, though it has varied to a slight extent, as I have
elsewhere described.
Some authors have maintained that the amount of variation in our
domestic productions is soon reached, and can never afterwards be
exceeded. It would be somewhat rash to assert that the limit has
been attained in any one case; for almost all our animals and plants
have been greatly improved in many ways within a recent period; and
this implies variation. It would be equally rash to assert that
characters now increased to their utmost limit, could not, after
remaining fixed for many centuries, again vary under new conditions of
life. No doubt, as Mr. Wallace has remarked with much truth, a limit
will be at last reached. For instance, there must be a limit to the
fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight of body to be carried, and the
power of contraction in the muscular fibres. But what concerns us is
that the domestic varieties of the same species differ from each other
in almost every character, which man has attended to and selected,
more than do the distinct species of the same genera. Isidore Geoffroy
St-Hilaire has proved this in regard to size, and so it is with colour
and probably with the length of hair. With respect to fleetness, which
depends on many bodily characters, Eclipse was far fleeter, and a
dray-horse is incomparably stronger than any two natural species
belonging to the same genus. So with plants, the seeds of the
different varieties of the bean or maize probably differ more in size,
than do the seeds of the distinct species in any one genus in the same
two families. The same remark holds good in regard to the fruit of the
several varieties of the plum, and still more strongly with the melon,
as well as in many other analogous cases.
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion,
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the
most important. Something, but how much we do not know, may be
attributed to the definite action of the conditions of life. Some,
perhaps a great, effect may be attributed to the increased use or
disuse of parts. The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species
appears to have played an important part in the origin of our
breeds. When several breeds have once been formed in any country,
their occasional intercrossing, with the aid of selection, has, no
doubt, largely aided in the formation of new sub-breeds; but the
importance of crossing has been much exaggerated, both in regard to
animals and to those plants which are propagated by seed. With
plants which are temporarily propagated by cuttings, buds, &c., the
importance of crossing is immense; for the cultivator may here
disregard the extreme variability both of hybrids and of mongrels, and
the sterility of hybrids; but plants not propagated by seed are of
little importance to us, for their endurance is only temporary. Over
all these causes of Change, the accumulative action of Selection,
whether applied methodically and quickly, or unconsciously and
slowly but more efficiently, seems to have been the predominant Power.
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