anything new, is there any other way in which selection becomes an agent in
evolution?

We can get some light on this question if we turn to what man has done with
his domesticated animals and plants. Through selection, i.e., artificial
selection, man has undoubtedly brought about changes as remarkable as any
shown by wild animals and plants. We know, moreover, a good deal about how
these changes have been wrought.

(1) By crossing different wild species or by crossing wild with races
already domesticated new combinations have been made. Parts of one
individual have been combined with parts of others, creating new
combinations. It is possible even that characters that are entirely new may
be produced by the interaction of factors brought into recombination.

(2) New characters appear from time to time in domesticated and in wild
species. These, like the mutants in Drosophila, are fully equipped at the
start. Since they breed true and follow Mendel's laws it is possible to
combine them with characters of the wild type or with those of other mutant
races.

Amongst the new mutant factors there may be some whose chief effect is on
the character that the breeder is already selecting. Such a modification
will be likely to attract attention. Superficially it may appear that the
factor for the original character has varied, while the truth may be that
another factor has appeared that has modified a character already present.
In fact, many or all Mendelian factors that affect the same organ may be
said to be modifiers of each other's effects. Thus the factor for vermilion
causes the eye to be one color, and the factor for eosin another color,
while eosin vermilion is different from both. Eosin may be said to be a
modifier of vermilion or vermilion of eosin. In general, however, it is
convenient to use the term "modifier" for cases in which the factor causes
a detectable change in a character already present or conspicuous.

[Illustration: FIG. 82. Scheme to indicate influence of the modifying
factors, cream and whiting. Neither produces any effect alone but they
modify other eye colors such as eosin.]

One of the most interesting, and at the same time most treacherous, kinds
of modifying factors is that which produces an effect _only_ when some
other factor is present. Thus Bridges has shown that there is a factor
called "cream" that does not affect the red color of the eye of the wild
fly, yet makes "eosin" much paler (fig