bility of the
stocks is poor. When flies with rudimentary wings are put into
competition with wild flies relatively few of the rudimentary flies come
through, especially if the culture is crowded. The hind legs are also
shortened. All of these effects are the results of a single
factor-difference." To be strictly accurate, then, one should not say
that a certain variation affects length of wing, but that its _chief_
effect is to shorten the wing.

"One may venture to guess," T. H. Morgan says,[47] "that some of the
specific and varietal differences that are characteristic of wild types
and which at the same time appear to have no survival value, are only
by-products of factors whose most important effect is on another part
of the organism where their influence is of vital importance."

"I am inclined to think," Professor Morgan continues, "that an
overstatement to the effect that each factor may affect the entire body,
is less likely to do harm than to state that each factor affects only a
particular character. The reckless use of the phrase 'unit character'
has done much to mislead the uninitiated as to the effects that a single
change in the germ-plasm may produce on the organism. Fortunately the
expression 'unit character' is being less used by those students of
genetics who are more careful in regard to the implications of their
terminology."

[Illustration: THE EFFECT OF ORTHODACTYLY

FIG. 17.--At the left is a hand with the third, fourth and
fifth fingers affected. The middle joints of these fingers are stiff and
cannot be bent. At the right the same hand is shown, closed. A normal
hand in the middle serves to illustrate by contrast the nature of the
abnormality, which appears in every generation of several large
families. It is also called symphalangism, and is evidently related to
the better-known abnormality of brachydactyly. Photograph from Frederick
N. Duncan.]

[Illustration: A FAMILY WITH ORTHODACTYLY

FIG. 18.--Squares denote males and circles females, as is usual
in the charts compiled by eugenists; black circles or squares denote
affected individuals. A1 had all fingers affected in the way shown in
Fig. 17; B2 had all but one finger affected; C2 had all but one finger
affected; D2 had all fingers affected; D3 has all but forefingers
affected. The family here shown is a branch, found by F. N. Duncan, of a
very large family first described by Harvey Cushing, in which this
abnormality has run for at