CHAPTER II

THE LAWS OF NATURAL INHERITANCE

MENDELISM

THE study of evolution involves the study of the ways in which the generations of plants and animals succeed each other; that is of the laws of inheritance when regarded from the point of view of science. Immense progress has been made in our knowledge of these laws during the last half-century, and the records of the investigations which have produced this result now fill many scores of volumes. Here all that will be attempted will be to give such a general account of the conclusions arrived at as may be just sufficient to enable a reader entirely uninstructed in biology to be able to understand the arguments concerning eugenic reforms. To make this sketch as brief as possible it will be necessary to be dogmatic, and in most cases to state results without describing the researches on which they are founded. As to any student of biology who should pick up this book, he is recommended at all events to omit this chapter.

From the above remarks it will be obvious that the number of persons who have done excellent work in genetics, that is, in the science of natural inheritance, is very great; so great that it is quite impossible here to mention all their names, whilst fair play makes it undesirable to pick out only a few as being worthy of special praise. Two of the pioneers in this field of research, however, stand out so conspicuously, at all events from the historical point of view, that to mention them alone involves no invidious comparisons with other workers; and these are Gregor Mendel and Francis Galton, both of whom by an odd coincidence were born in the year 1822.

These two great original thinkers advanced on different lines of inquiry, and following their example, or aided by their work, two schools of investigators have sprung into existence. The differences between these two schools are, in my opinion, now concerned with methods rather than with conclusions; for their broad final results seem to be in no way necessarily antagonistic. It may, however, be as well to treat them separately, and to give a brief historical account of the parallel growths which have sprung from the roots planted by these two great pioneers.

Dealing with Mendel in the first place, this patient Austrian monk carried on innumerable experiments with garden peas and other plants in his quiet monastery garden. His methods were excellent and his results unquestionable; but he lacked the power of forcing his work on the attention of men of science. His brief paper, which was published in 1865, lay buried until 1900, when it was rediscovered at a time when certain other investigators were beginning to tread close on his heels. He died in 1884 without having the remotest idea that 'Mendelism' would become a recognized word in scientific literature, and after altogether abandoning his scientific pursuits owing to the pressure of his religious duties.

The main underlying principle of Mendelism appears to be simple enough when it is fully realized, though thus to realize it does need some effort. As to the more recent developments of this branch of genetic research, they are far more complicated; but though from many points of view they are of great importance, yet they need not detain us here. Many methods of roughly illustrating the basic idea of Mendelism could no doubt be suggested; but here with that object in view, animals and plants will be compared to houses constructed in a somewhat peculiar manner. The main point to note in these imaginary houses is that the walls are always two bricks thick; and that every part of the building is similarly duplicated. Although in the case of man we know that both parents do not invariably die when a child is born, yet it will be convenient for the purposes of this analogy in the first instance to assume that the two parent houses

are always pulled down when material is needed for the construction of a new infant house. Now for the formation of a new generation, that is, for the building of such an infant house out of the old material, the rules of the game, if I may venture so to describe them, are as follows. To understand these rules it must, however, in the first place be noted that the bricks which form any pair of bricks, though not necessarily similar to each other, are sufficiently alike to make it always possible for either of them to be used in a corresponding position in a new building; and consequently that suitable bricks, taken from corresponding pairs of bricks in the parent houses, can always be obtained when building an infant house. For each pair of bricks in the infant house the choice is, in fact, always made from amongst four bricks, namely from those composing the two corresponding pairs in the parent house. And this choice is always made by selecting one brick by chance from the two bricks composing the corresponding pair in the father house, the other unused brick from that house being thrown away; whilst the second brick for the infant house is taken in a similar way from the mother house, with a similar waste of a brick in that case also. The double brick walls of the infant house are, in fact, built up from the double brick walls of the parent houses by chance selections from corresponding parts, the numbers of bricks supplied by the two parent houses being always equal.

This peculiar method of constructing a new house, a method which is followed generation after generation, entails certain consequences which are worth noting. If we consider the back history of a brick taken from the father house we see that it must, when that house was being built, have been taken either from the paternal grandfather house, or from the paternal grandmother house. And as we have seen that only one brick in a pair can be taken from the father house, it follows that a grandfather and a grandmother house on the same side can never both have supplied a brick for the make-up of any one pair of bricks. But as the selection of a brick from any parental pair of bricks is as a rule in each instance

quite fortuitous, if one brick in a pair in the infant house is derived from the paternal grandfather, another brick in a contiguous pair may well have been taken from the paternal grandmother. To put the matter in another way, as regards the characteristics dependent on a single pair of bricks, these bricks can only have been derived from two ancestors in any one generation however far back we may go; whilst some of the distant ancestors will have contributed none of the bricks utilized in building the new house. Before Mendel's discoveries, it was often thought that we might conceive all our ancestors as being, as it were, boiled down together, the concoction not being stirred enough to secure a perfect mixture, and that each one of us consists of a chance sample taken from this dish. In the light of Mendelism it seems, however, that we should rather regard ourselves as a mosaic consisting of little separable bits picked out by hazard from a large number of different ancestors; whilst as to certain remote ancestors, we really cannot claim any relationship whatever to them in a biological sense.

Thus far we have been considering the way in which an individual can claim descent from a chance assortment of many of his ancestors but not from all. But we must also consider the relationship between brothers and sisters; and for this purpose we must abandon the supposition that the parental houses are destroyed at each birth, and substitute the idea that each brick in the parental houses gives off a minute representation of a brick with which the same game can be played in a similar manner, these representations being capable in due season of growing into full-sized bricks. For simplicity we shall, however, continue to speak of bricks rather than of representations of bricks. Now we have seen that in any pair of bricks in the infant house, one must have been drawn by chance from the father house. But in the case of brothers, one brother may draw one of the two bricks forming a parental pair of bricks, and the other brother may draw the other brick; the chances being in fact even that they will not both draw the same brick. Since each infant house is composed of a great

DOMINANT QUALITIES

15

many pairs of bricks, it becomes practically certain that no two brothers will be exactly alike each other though, for certain reasons, all this may possibly not apply to twins. Roughly speaking, half the bricks used in the construction of any two brothers will be the same and half will be different; and they will consequently resemble each other to a considerable extent, whilst the differences between them will also be well marked. Then as to the resemblance between father and son, the son will draw half his bricks from the father house and half from the mother house; and consequently, looking to average results, the likeness between father and son will be somewhat the same as that existing between two brothers. Indeed, but for one circumstance not yet mentioned, this imaginary method of house-building would lead us to expect that the likenesses between the different members of the same family would be greater than they are in reality found to be; and this further complication must now be discussed.

Not only are we, as has been seen, built up of pairs of bricks, each such pair being instrumental in a certain way in building up our whole personality, but not infrequently the presence or absence of a particular kind of brick in a pair determines the presence or absence of a certain visible quality or character. Taking the case of a pair of bricks, one being black and the other blue, the rule of the game may be in this instance that one of these bricks, say the black one, is always placed as the outside brick of the house, it being therefore the only one which shows itself. This power which one kind of brick has of hiding (or partially hiding) another kind of brick is described as dominance, while the brick with the capacity for being hidden is described as being recessive. Now when a new generation is being formed from two parent houses in both of which this particular pair of bricks consists of a dominant black and a recessive blue, both houses consequently appearing black in this part of their structure, it may be that both the two black bricks will be thrown away, and both the two blue bricks retained to build the new infant house, which will therefore show