forms have
undergone. The characters by which domestic varieties differ from each
other are more {412} variable than those distinguishing species, though
hardly more so than with certain protean species; but this greater degree
of variability is not surprising, as varieties have generally been exposed
within recent times to fluctuating conditions of life, are much more liable
to have been crossed, and are still in many cases undergoing, or have
recently undergone, modification by man's methodical or unconscious
selection.

Domestic varieties as a general rule certainly differ from each other in
less important parts of their organisation than do species; and when
important differences occur, they are seldom firmly fixed; but this fact is
intelligible if we consider man's method of selection. In the living animal
or plant he cannot observe internal modifications in the more important
organs; nor does he regard them as long as they are compatible with health
and life. What does the breeder care about any slight change in the molar
teeth of his pigs, or for an additional molar tooth in the dog; or for any
change in the intestinal canal or other internal organ? The breeder cares
for the flesh of his cattle being well marbled with fat, and for an
accumulation of fat within the abdomen of his sheep, and this he has
effected. What would the floriculturist care for any change in the
structure of the ovarium or of the ovules? As important internal organs are
certainly liable to numerous slight variations, and as these would probably
be inherited, for many strange monstrosities are transmitted, man could
undoubtedly effect a certain amount of change in these organs. When he has
produced any modification in an important part, it has generally been
unintentionally in correlation with some other conspicuous part, as when he
has given ridges and protuberances to the skulls of fowls, by attending to
the form of the comb, and in the case of the Polish fowl to the plume of
feathers on the head. By attending to the external form of the
pouter-pigeon, he has enormously increased the size of the oesophagus, and
has added to the number of the ribs, and given them greater breadth. With
the carrier-pigeon, by increasing, through steady selection, the wattles on
the upper mandible, he has greatly modified the form of the lower mandible;
and so in many other cases. Natural species, on the other hand, have been
modified exclusively for their own