_The Evidence from Non-Disjunction_

In the course of the work on Drosophila exceptions appeared in one strain where certain individuals did not conform to the scheme of s.e.x linked inheritance. For a moment the hypothesis seemed to fail, but a careful examination led to the suspicion that in this strain something had happened to the s.e.x chromosomes. It was seen that if in some way the X chromosomes failed to disjoin in certain eggs, the exceptions could be explained. The a.n.a.lysis led to the suggestion that if the Y chromosome had got into the female line the results would be accounted for, since its presence there would be expected to cause this peculiar non-disjunction of the X chromosomes.

That this was the explanation was shown when the material was examined. The females that gave these results were found by Bridges to have two X's and a Y chromosome.

The normal chromosome group of the female is shown in figure 52 and the chromosome group of one of the exceptional females is shown in figure 69.

In a female of this kind there are three s.e.x chromosomes X X Y which are h.o.m.ologous in the sense that in normal individuals the two present are mates and separate at the reduction division. If in the X X Y individual X and X conjugate and separate at reduction and the unmated Y is free to move to either pole of the spindle, two kinds of mature eggs will result, viz., X and XY. If, on the other hand, X and Y conjugate and separate at reduction and the remaining X is free to go to either pole, four kinds of eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are expected: viz. many XY and X eggs and a few XX and Y eggs.

[Ill.u.s.tration: FIG. 69. Figure of the chromosome group of an XXY female, that gives non-disjunction.]

These four kinds of eggs may be fertilized either by female-producing sperms or male-producing sperms, as indicated in the diagram (fig. 70).

[Ill.u.s.tration: FIG. 70. Scheme showing the results of fertilizing white bearing eggs (4 kinds) resulting from non-disjunction. The upper half of the diagram gives the results when these eggs are fertilized by normal red bearing, female producing sperm, the lower half by normal, male producing sperm.]

If such an XXY female carried white bearing Xs (open X in the figures), and the male carried a red bearing X (black X in the figures) it will be seen that there should result an exceptional cla.s.s of sons that are red, and an exceptional cla.s.s of daughters that are white. Tests of these exceptions show that they behave subsequently in heredity as their composition requires. Other tests may also be made of the other cla.s.ses of offspring.

Bridges has shown that they fulfill all the requirements predicted. Thus a result that seemed in contradiction with the chromosome hypothesis has turned out to give a brilliant confirmation of that theory both genetically and cytologically.

HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL

In pa.s.sing I invite your attention to a speculation based on our maps of the chromosomes--a speculation which I must insist does not pretend to be more than a guess but has at least the interest of being the first guess that we have ever been in position to make as to how many factors go towards the makeup of the germ plasm.

We have found practically no factors less than .04 of a unit apart. If our map includes the entire length of the chromosomes and if we a.s.sume factors are uniformly distributed along the chromosome at distances equal to the shortest distance yet observed, viz. .04, then we can calculate roughly how many hereditary factors there are in Drosophila. The calculation gives about 7500 factors. The reader should be cautioned against accepting the above a.s.sumptions as strictly true, for crossing-over values are known to differ according to different environmental conditions (as shown by Bridges for age), and to differ even in different parts of the chromosome as a result of the presence of specific genetic factors (as shown by Sturtevant). Since all the chromosomes except the X chromosomes are double we must double our estimate to give the _total_ number of factors, but the half number is the number of the different kinds of factors of Drosophila.

CONCLUSIONS

I have pa.s.sed in review a long series of researches as to the nature of the hereditary material. We have in consequence of this work arrived within sight of a result that seemed a few years ago far beyond our reach. The mechanism of heredity has, I think, been discovered--discovered not by a flash of intuition but as the result of patient and careful study of the evidence itself.

With the discovery of this mechanism I venture the opinion that the problem of heredity has been solved. We know how the factors carried by the parents are sorted out to the germ cells. The explanation does not pretend to state how factors arise or how they influence the development of the embryo. But these have never been an integral part of the doctrine of heredity. The problems which they present must be worked out in their own field. So, I repeat, the mechanism of the chromosomes offers a satisfactory solution of the traditional problem of heredity.

CHAPTER IV

SELECTION AND EVOLUTION

Darwin's Theory of Natural Selection still holds today first place in every discussion of evolution, and for this very reason the theory calls for careful scrutiny; for it is not difficult to show that the expression "natural selection" is to many men a metaphor that carries many meanings, and sometimes different meanings to different men. While I heartily agree with my fellow biologists in ascribing to Darwin himself, and to his work, the first place in biological philosophy, yet recognition of this claim should not deter us from a careful a.n.a.lysis of the situation in the light of work that has been done since Darwin's time.

THE THEORY OF NATURAL SELECTION

In his great book on the _Origin of Species_, Darwin tried to do two things: first, to show that the evidence bearing on evolution makes that explanation probable. No such great body of evidence had ever been brought together before, and it wrought, as we all know, a revolution in our modes of thinking.

Darwin also set himself the task of showing _how_ evolution might have taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to everyone, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and to leave most offspring. In consequence their descendants should in time replace through compet.i.tion the less well-adapted individuals of the species. This is the process Darwin called natural selection, and Spencer the survival of the fittest.

Stated in these general terms there is nothing in the theory to which anyone is likely to take exception. But let us examine the argument more critically.

[Ill.u.s.tration: FIG. 71. Series of leaves of a tree arranged according to size. (After de Vries.)]

If we measure, or weigh, or cla.s.sify any character shown by the individuals of a population, we find differences. We recognize that some of the differences are due to the varied experiences that the individuals have encountered in the course of their lives, i.e. to their environment, but we also recognize that some of the differences may be due to individuals having different inheritances--different germ plasms. Some familiar examples will help to bring home this relation.

If the leaves of a tree are arranged according to size (fig. 71), we find a continuous series, but there are more leaves of medium size than extremes.

If a lot of beans be sorted out according to their weights, and those between certain weights put into cylinders, the cylinders, when arranged according to the size of the beans, will appear as shown in figure 72. An imaginary line running over the tops of the piles will give a curve (fig.

73) that corresponds to the curve of probability (fig. 74).

[Ill.u.s.tration: FIG. 72. Beans put into cylindrical jars according to the sizes of the beans. The jars arranged according to size of contained beans.

(After de Vries.)]

[Ill.u.s.tration: FIG. 73. A curve resulting from arrangement of beans according to size. (After de Vries.)]

If we stand men in lines according to their height (fig. 75) we get a similar arrangement.

[Ill.u.s.tration: FIG. 74. Curve of probability.]

[Ill.u.s.tration: FIG. 75. Students arranged according to size. (After Blakeslee.)]

The differences in size shown by the individual beans or by the individual men are due in part to heredity, in part to the environment in which they have developed. This is a familiar fact of almost every-day observation. It is well shown in the following example. In figure 76 the two boys and the two varieties of corn, which they are holding, differ in height. The pedigrees of the boys (fig. 77) make it probable that their height is largely inherited and the two races of corn are known to belong to a tall and a short race respectively. Here, then, the chief effect or difference is due to heredity. On the other hand, if individuals of the same race develop in a favorable environment the result is different from the development in an unfavorable environment, as shown in figure 78. Here to the right the corn is crowded and in consequence dwarfed, while to the left the same kind of corn has had more room to develop and is taller.

[Ill.u.s.tration: FIG. 76. A short and a tall boy each holding a stalk of corn--one stalk of a race of short corn, the other of tall corn. (After Blakeslee.)]

[Ill.u.s.tration: FIG. 77. Pedigree of boys shown in Fig. 76. (After Blakeslee.)]

Darwin knew that if selection of particular kinds of individuals of a population takes place the next generation is affected. If the taller men of a community are selected _the average_ of their offspring will be taller than the average of the former population. If selection for tallness again takes place, still taller men will _on the average_ arise. If, amongst these, selection again makes a choice the process would, he thought, continue (fig. 79).

[Ill.u.s.tration: FIG. 78. A race of corn reared under different conditions.]

We now recognize that this statement contains an important truth, but we have found that it contains only a part of the truth. Any one who repeats for himself this kind of selection experiment will find that while his average cla.s.s will often change in the direction of his selection, the process slows down as a rule rather suddenly (fig. 80). He finds, moreover, that the limits of variability are not necessarily transcended as the process continues even although the average may for a while be increased.

More tall men may be produced by selection of this kind, but the tallest men are not necessarily any taller than the tallest in the original population.

[Ill.u.s.tration: FIG. 79. Curves showing how (hypothetically) selection might be supposed to bring about progress in direction of selection. (After Goldschmidt.)]

Selection, then, has not produced anything new, but only more of certain kinds of individuals. Evolution, however, means producing more new things, not more of what already exists.

Darwin seems to have thought that the range of variation shown by the offspring of a given individual about that type of individual would be as wide as the range shown by the original population (fig. 79), but Galton's work has made it clear that this is not the case in a general or mixed population. If the offspring of individuals continued to show, as Darwin seems to have thought, as wide a range on each side of their parents' size, so to speak, as did the original population, then it would follow that selection could slide successive generations along in the direction of selection.

[Ill.u.s.tration: FIG. 80. Diagram ill.u.s.trating the results of selection for extra bristles in D. ampelophila. Selection at first produces decided effects which soon slow down and then cease. (MacDowell.)]

Darwin himself was extraordinarily careful, however, in the statements he made in this connection and it is rather by implication than by actual reference that one can ascribe this meaning to his views. His contemporaries and many of his followers, however, appear to have accepted this _sliding scale_ interpretation as the cardinal doctrine of evolution.

If this is doubted or my statement is challenged then one must explain why de Vries' mutation theory met with so little enthusiasm amongst the older group of zoologists and botanists; and one must explain why Johannsen's splendid work met with such bitter opposition from the English school--the biometricians--who amongst the post-Darwinian school are a.s.sumed to be the lineal descendants of Darwin.

And in this connection we should not forget that just this sort of process was supposed to take place in the inheritance of use and disuse. What is gained in one generation forms the basis for further gains in the next generation. Now, Darwin not only believed that acquired characters are inherited but turned more and more to this explanation in his later writings. Let us, however, not make too much of the matter; for it is much less important to find out whether Darwin's ideas were vague, than it is to make sure that our own ideas are clear.

If I have made several statements here that appear dogmatic let me now attempt to justify them, or at least give the evidence which seems to me to make them probable.

The work of the Danish botanist, Johannsen, has given us the most carefully a.n.a.lyzed case of selection that has ever been obtained. There are, moreover, special reasons why the material that he used is better suited to give definite information than any other so far studied. Johannsen worked with the common bean, weighing the seeds or else measuring them. These beans if taken from many plants at random give the typical curve of probability (fig. 74). The plant multiplies by self-fertilization. Taking advantage of this fact Johannsen kept the seeds of each plant separate from the others, and raised from them a new generation. When curves were made from these new groups it was found that some of them had different modes from that of the original general population (fig. 81 A-E, bottom group).

They are shown in the upper groups (A, B, C, D, E). But do not understand me to say that the offspring of each bean gave a different mode.

[Ill.u.s.tration: FIG. 81. Pure lines of beans. The lower figure gives the general population, the other figures give the pure lines within the population. (After Johannsen.)]

On the contrary, some of the lines would be the same.