A Critique of the Theory of Evolution

by Thomas Hunt Morgan

in which the wing pads did not expand (fig. 33). It was found that this peculiarity is shown in only about twenty per cent of the individuals supposed to inherit it. Later it was found that this stock lacked two bristles on the sides of the thorax. By means of this knowledge the heredity of the character was easily determined. It appears that while the expansion of the wing pads fails to occur once in five times—probably because it is an environmental effect peculiar to this stock,—yet the minute difference of the presence or absence of the two lateral bristles is a constant feature of the flies that carry this particular factor.

In the preceding cases I have spoken as though a factor influenced only one part of the body. It would have been more accurate to have stated that the chief effect of the factor was observed in a particular part of the body. Most students of genetics realize that a factor difference usually affects more than a single character. For example, a mutant stock called rudimentary wings has as its principle characteristic very short wings (fig. 34). But the factor for rudimentary wings also produces other

effects as well. The females are almost completely sterile, while the males are fertile. The viability of the stock 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.

Fig. 34. Mutant race of fruit fly, called rudimentary.

One may venture the guess 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.

It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whose chief influence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters.

I am inclined to think 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.