_Darwin_

Of the four great historical speculations about evolution, the doctrine of Natural Selection of Darwin and Wallace has met with the most widespread acceptance. In the last lecture I intend to examine this theory critically.

Here we are concerned only with its broadest aspects.

Darwin appealed to _chance variations_ as supplying evolution with the material on which natural selection works. If we accept, for the moment, this statement as the cardinal doctrine of natural selection it may appear that evolution is due, (1) _not_ to an _orderly_ response of the organism to its environment, (2) _not_ in the main to the activities of the animal through the use or disuse of its parts, (3) _not_ to any innate principle of living material itself, and (4) above all _not_ to purpose either from within or from without. Darwin made quite clear what he meant by chance. By chance he did not mean that the variations were not causal. On the contrary he taught that in Science we mean by chance only that the particular combination of causes that bring about a variation are not known. They are accidents, it is true, but they are causal accidents.

In his famous book on "Animals and Plants under Domestication", Darwin dwells at great length on the nature of the conditions that bring about variations. If his views seem to us today at times vague, at times problematical, and often without a secure basis, nevertheless we find in every instance, that Darwin was searching for the _physical causes of variation_. He brought, in consequence, conviction to many minds that there are abundant indications, even if certain proof is lacking, that the causes of variation are to be found in natural processes.

Today the belief that evolution takes place by means of natural processes is generally accepted. It does not seem probable that we shall ever again have to renew the old contest between evolution and special creation.

But this is not enough. We can never remain satisfied with a negative conclusion of this kind. We must find out what natural causes bring about variations in animals and plants; and we must also find out what kinds of variations are inherited, and how they are inherited. If the circ.u.mstantial evidence for organic evolution, furnished by comparative anatomy, embryology and paleontology is cogent, we should be able to observe evolution going on at the present time, i.e. we should be able to observe the occurrence of variations and their transmission. This has actually been done by the geneticist in the study of mutations and Mendelian heredity, as the succeeding lectures will show.

CHAPTER II

THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS

Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied the heredity of certain characters of the common edible pea, in the garden of the monastery at Brunn.

In his account of his work written in 1868, he said:

"It requires indeed some courage to undertake a labor of such a far-reaching extent; it appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be over-estimated in connection with the history of the evolution of organic forms."

He tells us also why he selected peas for his work:

"The selection of the plant group which shall serve for experiments of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results."

"The experimental plants must necessarily

1. Possess constant differentiating characters.

2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection."

Why do biologists throughout the world to-day agree that Mendel's discovery is one of first rank?

A great deal might be said in this connection. What is essential may be said in a few words. Biology had been, and is still, largely a descriptive and speculative science. _Mendel showed by experimental proof that heredity could be explained by a simple mechanism. His discovery has been exceedingly fruitful._

Science begins with nave, often mystic conceptions of its problems. It reaches its goal whenever it can replace its early guessing by verifiable hypotheses and predictable results. This is what Mendel's law did for heredity.

MENDEL'S FIRST DISCOVERY--SEGREGATION

[Ill.u.s.tration: FIG. 13. Diagram ill.u.s.trating a cross between a red (dark) and a white variety of four o'clock (Mirabilis jalapa).]

Let us turn to the demonstration of his first law--the law of segregation.

The first case I choose is not the one given by Mendel but one worked out later by Correns. If the common garden plant called four o'clock (Mirabilis jalapa) with red flowers is crossed to one having white flowers, the offspring are pink (fig. 13). The hybrid, then, is intermediate in the color of its flowers between the two parents. If these hybrids are inbred the offspring are white, pink and red, in the proportion of 1:2:1. All of these had the same ancestry, yet they are of three different kinds. If we did not know their history it would be quite impossible to state what the ancestry of the white or of the red had been, for they might just as well have come from pure white and pure red ancestors respectively as to have emerged from the pink hybrids. Moreover, when we test them we find that they are as pure as are white or red flowering plants that have had all white or all red flowering ancestors.

Mendel's Law explains the results of this cross as shown in figure 14.

The egg cell from the white parent carries the factor for white, the pollen cell from the red parent carries the factor for red. The hybrid formed by their union carries both factors. The result of their combined action is to produce flowers intermediate in color.

When the hybrids mature and their germ cells (eggs or pollen) ripen, each carries only one of these factors, either the red or the white, but not both. In other words, the two factors that have been brought together in the hybrid separate in its germ cells. Half of the egg cells are white bearing, half red bearing. Half of the pollen cells are white bearing, half red bearing. Chance combinations at fertilization give the three cla.s.ses of individuals of the second generation.

[Ill.u.s.tration: FIG. 14. Diagram ill.u.s.trating the history of the factors in the germ cells of the cross shown in Fig. 13.]

The white flowering plants should forever breed true, as in fact they do.

The red flowering plants also breed true. The pink flowering plants, having the same composition as the hybrids of the first generation, should give the same kind of result. They do, indeed, give this result i.e. one white to two pink to one red flowered offspring.

[Ill.u.s.tration: FIG. 15. Diagram ill.u.s.trating a cross between special races of white and black fowls, producing the blue (here gray) Andalusian.]

Another case of the same kind is known to breeders of poultry. One of the most beautiful of the domesticated breeds is known as the Andalusian. It is a slate blue bird shading into blue-black on the neck and back. Breeders know that these blue birds do not breed true but produce white, black, and blue offspring.

[Ill.u.s.tration: FIG. 16. Diagram showing history of germ cells of cross of Fig. 15. The larger circles indicate the color of the birds; their enclosed small circles the nature of the factors in the germ cells of such birds.]

The explanation of the failure to produce a pure race of Andalusians is that they are like the pink flowers of the four o'clock, i.e., they are a hybrid type formed by the meeting of the white and the black germ cells. If the whites produced by the Andalusians are bred to the blacks (both being pure strains), all the offspring will be blue (fig. 15); if these blues are inbred they will give 1 white, to 2 blues, to 1 black. In other words, the factor for white and the factor for black separate in the germ cells of the hybrid Andalusian birds (fig. 16).

[Ill.u.s.tration: FIG. 17. Diagram of Mendel's cross between yellow (dominant) and green (recessive) peas.]

The third case is Mendel's cla.s.sical case of yellow and green peas (fig.

17). He crossed a plant belonging to a race having yellow peas with one having green peas. The hybrid plants had yellow seeds. These hybrids inbred gave three yellows to one green. The explanation (fig. 18) is the same in principle as in the preceding cases. The only difference between them is that the hybrid which contains both the yellow and the green factors is in appearance not intermediate, but like the yellow parent stock. Yellow is said therefore to be dominant and green to be recessive.

[Ill.u.s.tration: FIG. 18. Diagram ill.u.s.trating the history of the factors in the cross shown in Fig. 17.]

Another example where one of the contrasted characters is dominant is shown by the cross of Drosophila with vestigial wings to the wild type with long wings (fig. 19). The F_1 flies have long wings not differing from those of the wild fly, so far as can be observed. When two such flies are inbred there result three long to one vestigial.

[Ill.u.s.tration: FIG. 19. Diagram ill.u.s.trating a cross between a fly (Drosophila ampelophila) with long wings and a mutant fly with vestigial wings.]

The question as to whether a given character is dominant or recessive is a matter of no theoretical importance for the principle of segregation, although from the notoriety given to it one might easily be misled into the erroneous supposition that it was the discovery of this relation that is Mendel's crowning achievement.

Let me ill.u.s.trate by an example in which the hybrid standing between two types overlaps them both. There are two mutant races in our cultures of the fruit fly Drosophila that have dark body color, one called sooty, another which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives offspring that are intermediate in color. Some of them are so much like sooty that they cannot be distinguished from sooty. At the other extreme some of the hybrids are as dark as the lightest of the ebony flies. If these hybrids are inbred there is a continuous series of individuals, sooties, intermediates and ebonies. Which color here shall we call the dominant? If the ebony, then in the second generation we count three ebonies to one sooty, putting the hybrids with the ebonies. If the dominant is the sooty then we count three sooties to one ebony, putting the hybrids with the sooties. The important fact to find out is whether there actually exist three cla.s.ses in the second generation. This can be ascertained even when, as in this case, there is a perfectly graded series from one end to the other, by testing out individually enough of the flies to show that one-fourth of them never produce any descendants but ebonies, one-fourth never any but sooties, and one-half of them give rise to both ebony and sooty.

[Ill.u.s.tration: FIG. 20. Cross between two allelomorphic races of Drosophila, sooty and ebony, that give a completely graded series in F_2.]

MENDEL'S SECOND DISCOVERY--INDEPENDENT a.s.sORTMENT

Besides his discovery that there are pairs of characters that disjoin, as it were, in the germ cells of the hybrid (law of segregation) Mendel made a second discovery which also has far-reaching consequences. The following case ill.u.s.trates Mendel's second law.

If a pea that is yellow and round is crossed to one that is green and wrinkled (fig. 21), all of the offspring are yellow and round. Inbred, these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green wrinkled. All the yellows taken together are to the green as 3:1. All the round taken together are to the wrinkled as three to one; but some of the yellows are now wrinkled and some of the green are now round. There has been a recombination of characters, while at the same time the results, for each pair of characters taken separately, are in accord with Mendel's Law of Segregation, (fig. 22). The second law of Mendel may be called the law of independent a.s.sortment of different character pairs.

[Ill.u.s.tration: FIG. 21. Cross between yellow-round and green-wrinkled peas, giving the 9: 3: 3: 1 ratio in F_2.]

We can, as it were, take the characters of one organism and recombine them with those of a different organism. We can explain this result as due to the a.s.sortment of factors for these characters in the germ cells according to a definite law.

[Ill.u.s.tration: FIG. 22. Diagram to show the history of the factor pairs yellow-green and round-wrinkled of the cross in Fig. 21.]

As a second ill.u.s.tration let me take the cla.s.sic case of the combs of fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23), the offspring have a comb different from either. It is called a walnut comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1 single. This proportion shows that the grandparental types differed in respect to two pairs of characters.