Part 3 (1/2)

[Ill.u.s.tration: FIG. 23. Cross between pea and rose combed fowls. (Charts of Baur and Goldschmidt.)]

A fourth case is shown in the fruit fly, where an ebony fly with long wings is mated to a grey fly with vestigial wings (fig. 24). The offspring are gray with long wings. If these are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).

[Ill.u.s.tration: FIG. 24. Cross between long ebony and gray vestigial flies.]

The possibility of interchanging characters might be ill.u.s.trated over and over again. It is true not only when two pairs of characters are involved, but when three, four, or more enter the cross.

[Ill.u.s.tration: FIG. 25. Diagram to show the history of the factors in the cross shown in Fig. 24.]

It is as though we took individuals apart and put together parts of two, three or more individuals by subst.i.tuting one part for another.

Not only has this power to make whatever combinations we choose great practical importance, it has even greater theoretical significance; for, it follows that the individual is not in itself the unit in heredity, but that within the germ-cells there exist smaller units concerned with the transmission of characters.

The older mystical statement of the individual as a unit in heredity has no longer any interest in the light of these discoveries, except as a past phase of biological history. We see, too, more clearly that the sorting out of factors in the germ plasm is a very different process from the influence of these factors on the development of the organism. There is today no excuse for confusing these two problems.

If mechanistic principles apply also to embryonic development then the course of development is capable of being stated as a series of chemico-physical reactions and the ”_individual_” is merely a term to express the sum total of such reactions and should not be interpreted as something different from or more than these reactions. So long as so little is known of the actual processes involved in development the use of the term ”individuality”, while giving the appearance of profundity, in reality often serves merely to cover ignorance and to make a mystery out of a mechanism.

THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.

Darwin based many of his conclusions concerning variation and heredity on the evidence derived from the garden and from the stock farm. Here he was handicapped to some extent, for he had at times to rely on information much of which was uncritical, and some of which was worthless.

Today we are at least better informed on _two_ important points; one concerning the _kinds_ of variations that furnish to the cultivator the materials for his selection; the other concerning the modes of inheritance of these variations. We know now that new characters are continually appearing in domesticated as well as in wild animals and plants, that these characters are often sharply marked off from the original characters, and whether the differences are great or whether they are small they are transmitted alike according to Mendel's law.

Many of the characteristics of our domesticated animals and cultivated plants originated long ago, and only here and there have the records of their first appearance been preserved. In only a few instances are these records clear and definite, while the complete history of any large group of our domesticated products is unknown to us.

Within the last five or six years, however, from a common wild species of fly, the fruit fly, Drosophila ampelophila, which we have brought into the laboratory, have arisen over a hundred and twenty-five new types whose origin is completely known. Let me call attention to a few of the more interesting of these types and their modes of inheritance, comparing them with wild types in order to show that the kinds of inheritance found in domesticated races occur also in wild types. The results will show beyond dispute that the characters of wild types are inherited in precisely the same way as are the characters of the mutant types--a fact that is not generally appreciated except by students of genetics, although it is of the most far-reaching significance for the theory of evolution.

A mutant appeared in which the eye color of the female was different from that of the male. The eye color of the mutant female is a dark eosin color, that of the male yellowish eosin. From the beginning this difference was as marked as it is to-day. Breeding experiments show that eosin eye color differs from the red color of the eye of the wild fly by a single mutant factor. Here then at a single step a type appeared that was s.e.xually dimorphic.

Zoologists know that s.e.xual dimorphism is not uncommon in wild species of animals, and Darwin proposed the theory of s.e.xual selection to account for the difference between the s.e.xes. He a.s.sumed that the male preferred certain kinds of females differing from himself in a particular character, and thus in time through s.e.xual selection, the s.e.xes came to differ from each other.

[Ill.u.s.tration: FIG. 26. Clover b.u.t.terfly (Colias philodice) with two types of females, above; and one type of male, below.]

In the case of eosin eye color no such process as that postulated by Darwin to account for the differences between the s.e.xes was involved; for the single mutation that brought about the change also brought in the dimorphism with it.

In recent years zoologists have carefully studied several cases in which two types of female are found in the same species. In the common clover b.u.t.terfly, there is a yellow and a white type of female, while the male is yellow (fig. 26). It has been shown that a single factor difference determines whether the female is yellow or white. The inheritance is, according to Gerould, strictly Mendelian.

[Ill.u.s.tration: FIG. 27. Papilio turnus with two types of females above and one type of male below.]

In Papilio turnus there exist, in the southern states, two kinds of females, one yellow like the male, one black (fig. 27). The evidence here is not so certain, but it seems probable that a single factor difference determines whether the female shall be yellow or black.

Finally in Papilio polytes of Ceylon and India three different types of females appear, (fig. 28 to right) only one of which is like the male. Here the a.n.a.lysis of the breeding data shows the possibility of explaining this case as due to two pairs Mendelian factors which give in combination the three types of female.

[Ill.u.s.tration: FIG. 28. Papilio polytes, with three types of female to right and one type of male above to left.]

Taking these cases together, they furnish a much simpler explanation than the one proposed by Darwin. They show also that characters like these shown by wild species may follow Mendel's law.

[Ill.u.s.tration: FIG. 29. Mutant race of fruit fly with intercalated duplicate mesothorax on dorsal side.]

There has appeared in our cultures a fly in which the third division of the thorax with its appendages has changed into a segment like the second (fig.

29). It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference.