one half hybrid blues and one half whites--no black at all.
If the cross had been between black hybrid guinea pigs and white
recessive specimens the result would have been half hybrid blacks and
half pure whites.

Or supposing the mating to have occurred between the pure dominant
(black) and the hybrid the result would have been, in the fowls half
pure black and half hybrid blue; in the guinea pig all the progeny
would have been black, half pure blacks and half hybrid blacks.

Germ cells of first parent (black or dominant) _B_ + _B_
Germ cells of second parent (hybrid) _B_ + _W_
-------------
_BB_ + _BB_
_BW_ + _BW_
----------------------
_2BB_ + _2BW_

In the case of the guinea pigs, although the progeny all look alike
(black) their history would show that they were fundamentally unlike,
for if crossed with white again the result would be the production of
all black looking guinea pigs from the cross with the _BB_ forms, and
half black and half white from the _BW_ cross.

On account of the fact of variation every individual is in a certain
sense a hybrid. One's two parents have the species characters in
common but there are certain distinctive traits that hybridize and
follow Mendel's law of heredity. By no means is it to be understood
that all individual distinctive traits follow this rule in heredity.
Many individual characteristics are what we have learned to call
fluctuations--small deviations above or below an average condition of
a group. Such differences play no part in Mendelian heredity. Other
characteristics may be bodily modifications resulting from the direct
reaction between the body tissues and the environing conditions; such
traits would not be represented in the organization of the germ cells
and consequently would not be inherited at all. At present it seems
that the only characteristics that "Mendelize" are those known as
"unit characters." Such characters seem to have their origin in real
variations or mutations and though each may show fluctuations, these
fluctuations in themselves are not hereditary.

This conception of the unit character is an extremely important
element in the whole Mendelian theory and it has extended be