Applied Eugenics Part 40

One of the most interesting results is that there were obtained among these first 200 individuals studied no p.r.o.nounced blonds, although the ancestry is North European, where blondness is more or less prevalent.[200] The exact distribution is:

Male Female

Light-brown 12% 16% Medium-brown to dark 77 68 Very dark 11 6 Golden-red and red 0 10

Dr. Hrdlicka's cla.s.sification of the eye is as follows:

Male Female Gray 2% 4% Greenish 7 10 Blues 54 50 Browns 37 36

The head among Old Americans is in many cases notable for its good development, particularly in males. Among 12 groups of male immigrants[201] measured at Ellis Island under Dr. Hrdlicka's direction in recent years, not one group quite equals in this respect the Americans, the nearest approach being noted in the Irish, Bohemians, English, Poles, and North Italians. The type of head, however, differs among the Americans very widely, as is the case with most civilized races at the present day.

Head form is most conveniently expressed by means of the cephalic index, that is, the ratio of breadth to length. Anthropologists generally speak of any one with an index of 75 (or where the breadth is 75% of the length) and below this as dolichocephalic, or long-headed; from 75 to 80 is the cla.s.s of the mesocephalic, intermediates; while above 80 is that of the subbrachycephalic and brachycephalic, or round-headed. For the most part, the Old Americans fall into the intermediate cla.s.s, the average index of males being 78.3 and that of females 79.5.

Barring a few French Huguenots, the Old Americans considered here are mostly of British ancestry, and their head form corresponds rather closely to that of the English of the present day. In England, as is well known, the round-headed type of Central and Eastern Europe, the Alpine or Celto-Slav type, has few representatives. The population is composed princ.i.p.ally of long-headed peoples, deriving from the two great European stocks, the Nordic and the Mediterranean. To the latter the frequency of dark hair and brown eyes is probably due, both in England and America.

While the average of the Old Americans corresponds closely to the average of the English, there is a great deal of variation in both countries. Unfortunately, it is impossible to compare the present Americans with their ancestors, because measurements of the latter are lacking. But to a.s.sume that the early colonists did not differ greatly from the modern English is probably justifiable. A comparison of modern Americans (of the old white stock) with modern English should give basis for an opinion as to whether the English stock underwent any marked modifications, on coming to a new environment.

It has already been noted that the average cephalic index is practically the same; the only possibility of a change then lies in the amount of variability. Is the American stock more or less variable? Can a "melting pot" influence be seen, tending to produce h.o.m.ogeneity, or has change of environment rather produced greater variability, as is sometimes said to be the case?

The amount of variability is most conveniently measured by a coefficient known as the standard deviation ([Greek: s]), which is small when the range of variation is small, but large when diversity of material is great. The following comparisons of the point at issue may be made.[202]

Avg. [Greek: s]

100 American men 78.3 3.1 1011 Cambridge graduates (English males) 79.85 2.95

For the men, little difference is discernible. The Old Americans are slightly more long-headed than the English, but the amount of variation in this trait is nearly the same on the two sides of the ocean.

The average of the American women is 79.5 with [Greek: s] = 2.6. No suitable series of English women has been found for comparison.(203) It will be noted that the American women are slightly more round-headed than the men; this is found regularly to be the case, when comparisons of the head form of the two s.e.xes are made in any race.

In addition to establishing norms or standards for anthropological comparison, the main object of Dr. Hrdlicka's study was to determine whether the descendants of the early American settlers, living in a new environment and more or less constantly intermarrying, were being amalgamated into a distinct sub-type of the white race. It has been found that such amalgamation has not taken place to any important degree. The persistence in heredity of certain features, which run down even through six or eight generations, is one of the remarkable results brought out by the study.

If the process could continue for a few hundred years more, Dr.

Hrdlicka thinks, it might reach a point where one could speak of the members of old American families as of a distinct stock. But so far this point has not been reached; the Americans are almost as diverse and variable, it appears, as were their first ancestors in this country.

APPENDIX D

THE ESSENCE OF MENDELISM

It is half a century since the Austrian monk, Gregor Mendel, published in a provincial journal the results of his now famous breeding experiments with garden peas. They lay unnoticed until 1900, when three other breeders whose work had led them to similar conclusions, almost simultaneously discovered the work of Mendel and gave it to the world.

Breeding along the lines marked out by Mendel at once became the most popular method of attack, among those who were studying heredity. It became an extremely complicated subject, which can not be grasped without extended study, but its fundamentals can be briefly summarized.

Inherited differences in individuals, it will be admitted, are due to differences in their germ-plasms. It is convenient to think of these differences in germ-plasms (that is, differences in heredity) as being due to the presence in the germ-plasm of certain hypothetical units, which are usually referred to as factors. The factor, nowadays, is the ultimate unit of Mendelian research. Each of these factors is considered to be nearly or quite constant,--that is, it undergoes little, or no change from generation to generation. It is ordinarily resistant to "contamination" by other factors with which it may come in contact in the cell. The first fundamental principle of Mendelism, then, is the existence of relatively constant units, the Mendelian factors, as the basis for transmission of all the traits that go to make up an animal or plant.

Experimental breeding gives reason to believe that each factor has one or more alternatives, which may take its place in the mechanism of heredity, thereby changing the visible character of the individual plant or animal in which it occurs. To put the matter a little differently, one germ-cell differs from another in having alternatives present in place of some of the factors of the latter. A given germ-cell can never have more than one of the possible alternatives of each factor. These alternatives of a factor are called its allelomorphs.

Now a mature germ-cell has a single system of these factors: but when two germ-cells unite, there result from that union two kinds of cells--namely, immature germ-cells and body-cells; and both these kinds of cells contain a double system of factors, because of course they have received a single entire system from each parent. This is the second of the fundamental principles of Mendelism: that the factors are single in the mature germ-cell, but in duplicate in the body-cell (and also in the immature germ-cell).

In every cell with a double system of factors, there are necessarily present two representatives from each set of allelomorphs, but these may or may not be alike--or in technical language the individual may be h.o.m.ozygous, or heterozygous, as regards the given set of alternative factors. Looking at it from another angle, there is a single visible character in the plant or animal, but it is produced by a double factor, in the germ-plasm.

When the immature germ-cell, with its double system of factors, matures, it throws out half the factors, retaining only a single system: and the allelomorphic factors which then segregate into different cells are, as has been said above, ordinarily uninfluenced by their stay together.

But the allelomorphic factors are not the only ones which are segregated into different germ-cells, at the maturation of the cell; for the factors which are not alternative are likewise distributed, more or less independently of each other, so that it is largely a matter of chance whether factors which enter a cross in the same germ-cell, segregate into the same germ-cell or different ones, in the next generation. This is the next fundamental principle of Mendelism, usually comprehended under the term "segregation," although, as has been pointed out, it is really a double process, the segregation of alternative factors being a different thing from the segregation of non-alternative factors.

From this fact of segregation, it follows that as many kinds of germ-cells can be formed by an individual, as there are possible combinations of factors, on taking one alternative from each pair of allelomorphs present. In practice, this means that the possible number of different germ-cells is almost infinitely great, as would perhaps be suspected by anyone who has tried to find two living things that are just alike.

[Ill.u.s.tration: THE CARRIERS OF HEREDITY

FIG. 46.--Many different lines of study have made it seem probable that much, although not all, of the heredity of an animal or plant is carried in the nucleus of the germ-cell and that in this nucleus it is further located in little rods or threads which can be easily stained so as to become visible, and which have the name of chromosomes. In the above ill.u.s.tration four different views of the nucleus of the germ-cell of an earthworm are shown, with the chromosomes in different stages; in section 19 each chromosome is doubled up like a hairpin. Study of the fruit-fly Drosophila has made it seem probable not only that the hypothetical factors of heredity are located in the chromosomes, but that each factor has a perfectly definite location in its chromosome; and T. H. Morgan and his a.s.sociates have worked out an ingenious method of measuring the distance from either end, at which the factor lies. Photomicrograph after Foot and Strobell.]

Such is the essence of Mendelism; and the reader is probably ready to admit that it is not a simple matter, even when reduced to the simplest terms. To sum up, the princ.i.p.al features at the base of the hypothetical structure are these:

1. There exist relatively constant units in the germ-plasm.

2. There are two very distinct relationships which these units may show to each other. Two (or more) unit factors may be alternatives in the mechanism of inheritance, indicating that one is a variation (or loss) of the other; or they may be independent of each other in the mechanism of inheritance.

3. The mature germ-cell contains a single system of independent factors (one representative from each set of alternates).

The immature germ-cells, and body-cells, have double systems of independent factors (two from each set of alternatives).

4. The double system arises simply from the union of two single systems (i. e., two germ-cells), without union or even contamination of the factors involved.

In the formation of a single system (mature germ-cells) from a double (immature germ-cells), pairs of alternates separate, pa.s.sing into different germ-cells. Factors not alternates may or may not separate--the distribution is largely a matter of chance.

Such are the fundamental principles of Mendelism; but on them was early grafted a theoretical structure due mainly to the German zoologist, August Weismann. To understand his part in the story, we must advert to that much mooted and too often misunderstood problem furnished by the chromosomes. (See Fig. 46.) These little rods of easily stained material, which are found in every cell of the body, were picked out by Professor Weismann as the probable carriers of heredity. With remarkable acuteness, he predicted their behavior at cell-division, the intricate nature of which is usually the despair of every beginner in biology.

When Mendelian breeding, in the early years of this century, showed temporary pairing and subsequent separation of units in the germ-cell, it was soon realized that the observed facts of breeding fitted to a nicety the observed facts (predicted by Weismann) of chromosome-behavior; for at each cell-division the chromosomes, too, pair and separate again. The observed behavior of transmitted characters in animals and plants followed, in so many cases, the observed behavior of the chromosomes, that many students found it almost impossible to believe that there was no connection between the two, and Dr. Weismann's prediction, that the chromosomes are the carriers of heredity, came to be looked on as a fact, by many biologists.

But when so much of Professor Weismann's system was accepted, other parts of it went along, including a hypothetical system of "determiners"

in the chromosome, which were believed to determine the development of characters in the organism. Every trait of an animal or plant, it was supposed, must be represented in the germ-plasm by its own determiner; one trait, one determiner. Did a notch in the ear run through a pedigree? Then it must be due to a determiner for a notch in the ear in the germ-plasm. Was mathematical ability hereditary? Then there must be a determiner, the expression of which was mathematical ability.

For a while, this hypothesis was of service in the development of genetics; some students even began to forget that it was a hypothesis, and to talk as if it were a fact. But the exhaustive tests of experimental breeding of plants and animals have long caused most of the advanced students of genetics to drop this simple hypothesis.

In its place stands the factorial hypothesis, evolved by workers in America, England, and France at about the same time. As explained in Chapter V, this hypothesis carries the a.s.sumption that every visible character is due to the effects of not one but many factors in the germ-cell.

In addition to these fundamentals, there are numerous extensions and corollaries, some of them of a highly speculative nature. The reader who is interested in pursuing the subject farther must turn to one of the text-books on Mendelism.

In plant-breeding a good deal of progress has been made in the exact study of Mendelian heredity; in animal breeding, somewhat less; in human heredity, very little. The reason is obvious: that experiments can not be made in man, and students must depend on the results of such matings as they can find; that only a very few offspring result from each mating; and that generations are so long that no one observer can have more than a few under his eyes. These difficulties make Mendelian research in man a very slow and uncertain matter.

Altogether, it is probable that something like a hundred characters in man have been pointed out as inherited in Mendelian fashion. A large part of these are pathological conditions or rare abnormalities.

But the present writers can not accept most of these cases. It has been pointed out in Chapter V that there are good reasons for doubting that feeble-mindedness is inherited in a simple Mendelian fashion, although it is widely accepted as such. We can not help feeling that in most cases heredity in man is being made to appear much simpler than it really is; and that particularly in mental characters, a.n.a.lysis of traits has by no means reached the bottom.

If we were asked to make out a list of characters, as to the Mendelian inheritance of which there could be little doubt, we would hardly be able to go farther than the following:

The s.e.x-linked characters (one kind of color-blindness, hemophilia, one kind of night-blindness, atrophy of the optic nerve, and a few other rare abnormalities).