gle
modification of Mendel's original views which has arisen out of more ample
knowledge.

[Illustration: FIG. 2.

A wing feather and a contour feather of an ordinary and a silky fowl. The
peculiar ragged appearance of the silky feathers is due to the absence of
the little hooks or barbules which hold the barbs together. The silky
condition is recessive.]

As we have already seen, Mendel considered that in the gamete there was
either a definite something {31} corresponding to the dominant character or
a definite something corresponding to the recessive character, and that
these somethings whatever they were could not coexist in any single gamete.
For these somethings we shall in future use the term FACTOR. The factor,
then, is what corresponds in the gamete to the UNIT-CHARACTER that appears
in some shape or other in the development of the zygote. Tallness in the
pea is a unit-character, and the gametes in which it is represented are
said to contain the factor for tallness. Beyond their existence in the
gamete and their mode of transmission we make no suggestion as to the
nature of these factors.

[Illustration: FIG. 3.

Two double and an ordinary single primula flower. This form of double is
recessive to the single.]

{32}

[Illustration: FIG. 4.

Fowls' combs. A, pea; B, rose; C, single; D, walnut.]

On Mendel's view there was a factor corresponding to the dominant character
and another factor corresponding to the recessive character of each
alternative pair of unit-characters, and the characters were alternative
because no gamete could carry more than one of the two factors belonging to
the alternative pair. On the other hand, Mendel supposed that it always
carried either one or the other of such a pair. As experimental work
proceeded, {33} it soon became clear that there were cases which could not
be expressed in terms of this conception. The nature of the difficulty and
the way in which it was met will perhaps be best understood by considering
a set of experiments in which it occurred. Many of the different breeds of
poultry are characterised by a particular form of comb, and in certain
cases the inheritance of these has been carefully worked out. It was shown
that the rose comb (Fig. 4, B) with its flattened papillated upper surface
and backwardly projecting pike was dominant in the ordinary way to the
deeply serrated high single comb (Fig. 4, C) which is characteristic of the
Mediterranean races. Expe