The head among Old Americans is in many cases notable for its good development, particularly in males. Among 12 groups of maleimmigrants[201]measured at Ellis Island under Dr. Hrdlička'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 class 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 class, 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 principally 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 assume 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 homogeneity, 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 σ, 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]
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 σ = 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 sexes are made in any race.
In addition to establishing norms or standards for anthropological comparison, the main object of Dr. Hrdlička'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. Hrdlička 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.
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 homozygous, 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.
THE CARRIERS OF HEREDITYTHE CARRIERS OF HEREDITY
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 thesimplest terms. To sum up, the principal 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, passing 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 zoölogist, 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 assumption 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 thatin most cases heredity in man is being made to appear much simpler than it really is; and that particularly in mental characters, analysis 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 sex-linked characters (one kind of color-blindness, hemophilia, one kind of night-blindness, atrophy of the optic nerve, and a few other rare abnormalities).
Albinism. This appears to be a recessive, but probably involves multiple allelomorphs in man, as in other animals.
Brachydactyly, apparently a dominant. This is so much cited in text-books on Mendelism that the student might think it is a common character. As a fact, it is extremely rare, being found in only a few families. The similar trait of orthodactyly or symphalangism, which likewise appears to be a good Mendelian dominant, seems to exist in only one family. Traits like these, which are easily defined and occur very rarely, make up a large part of the cases of probably Mendelian heredity. They are little more than curiosities, their rarity and abnormal nature depriving them of evolutionary significance other than to demonstrate that Mendelian heredity does operate in man.
White blaze in the hair or, as it might better be called to show its resemblance to the trait found in other mammals, piebaldism. A rather rare dominant.[204]
Huntington's Chorea, which usually appears to be a good dominant, although the last investigators (Muncey and Davenport) found some unconformable cases.
A few abnormalities, such as a premature graying of the hair (one family cited by K. Pearson) are well enough attested to be admitted. Many others, such as baldness, are probably Mendelian but not yet sufficiently supported by evidence.
None of these characters, it will be observed, is of much significance eugenically. If the exact manner of inheritance of some of the more important mental and physical traits were known, it would be ofvalue. But it is not a prerequisite for eugenic action. Enough is known for a working program.
To sum up: the features in the modern view of heredity, which the reader must keep in mind, are the following:
1. That the various characters which make up the physical constitution of any individual plant or animal are due to the action (concurrently with the environment, of course) of what are called, for convenience, factors, separable hypothetical units in the germ-plasm, capable of independent transmission.
2. That each visible character is due to the coöperative action of an indefinitely large number of factors; conversely, that each of these factors affects an indefinitely large number of characters.
The most complete bibliography is that published by the State Board of Charities of the State of New York (Eugenics and Social Welfare BulletinNo. III, pp. 130, Albany, 1913).
An interesting historical review of eugenics, with critical comments on the literature and a bibliography of 100 titles, was published by A. E. Hamilton in thePedagogical Seminary, Vol. XXI, pp. 28-61, March, 1914.
Much of the important literature of eugenics has been mentioned in footnotes. For convenience, a few of the books which are likely to be most useful to the student are here listed:
Genetics and Eugenics, by W. E. Castle. Harvard University Press, Cambridge, 1916.
Heredity and Environment in the Development of Men, by Edwin G. Conklin. Princeton University Press, 1915.
Heredity in Relation to Eugenics, by C. B. Davenport, Henry Holt and Co., New York, 1911.
Essays in Eugenics, by Francis Galton. Eugenics Education Society, London, 1909.
Being Well-Born,by Michael F. Guyer. Indianapolis, Bobbs-Merrill Co., 1916.
The Social Direction of Human Evolution, by W. E. Kellicott. New York, 1911.
The Physical Basis of Society, by Carl Kelsey. New York, D. Appleton & Co., 1916.
Eugenics, by Edward Schuster. Collins' Clear Type Press, London and Glasgow, 1913.
Heredity, by J. Arthur Thompson. Edinburgh, 1908.
Genetics, by Herbert E. Walter. The Macmillan Co., New York, 1913.
An Introduction to Eugenics, by W. C. D. Whetham and C. D. Whetham. Macmillan and Co., London, 1912.
Heredity and Society, by W. C. D. Whetham and C. D. Whetham. Longmans, Green & Co., London, 1912.
The Family and the Nation, by W. C. D. Whetham and C. D. Whetham. Longmans, Green & Co., London, 1909.
The publications of the Galton Laboratory of National Eugenics, University of London, directed by Karl Pearson, and of the Eugenics Record Office, Cold Spring Harbor, Long Island, N. Y., directed by C. B. Davenport, furnish a constantly increasing amount of original material on heredity.
The principal periodicals are theJournal of Heredity(organ of the American Genetic Association), 511 Eleventh St., N. W., Washington, D. C. (monthly); and theEugenics Review(organ of the Eugenics Education Society), Kingsway House, Kingsway, W. C., London (quarterly). These periodicals are sent free to members of the respective societies. Membership in the American organization is $2 a year, in the English 1 guinea a year, associate membership 5 shillings a year.
Acquired Character, a modification of a germinal trait after cell fusion. It is difficult to draw a line between characters that are acquired and those that are inborn. The idea involved is as follows: in a standard environment, a given factor in the germ-plasm will develop into a trait which varies not very widely about a certain mean. The mean of this trait is taken as representing the germinal trait in its typical condition. But if the environment be not standard, if it be considerably changed, the trait will develop a variation far from the mean of that trait in the species. Thus an American, whose skin in the standard environment of the United States would be blonde, may under the environment of Cuba develop into a brunette. Such a wide variation from the mean thus caused is called an acquired character; it is usually impressed on the organism after the germinal trait has reached a full, typical development.
Allelomorph(one another form), one of a pair of factors which are alternative to each other in Mendelian inheritance. Instead of a single pair, there may be a group of "multiple allelomorphs," each member being alternative to every other member of the group.
Allelomorphism, a relation between two or more factors, such that two which are present in one zygote do not both enter into the same gamete, but are separated into sister gametes.
Biometry(life measure), the study of biology by statistical methods.
Brachydactyly(short-finger), a condition in which the bones, particularly of the fingers and toes, fail to grow to their normal length. In well-marked cases one of these is a reduction from three phalanges or joints to two.
Character(a contraction of "characteristic"), a term which is used, often rather vaguely, to designate any function, feature, or organ of the body or mind.
Chromosome(color body, so called from its affinity for certain stains), a body of peculiar protoplasm, in the nucleus of the cell. Each species has its own characteristic number; the cells of the human body contain 24 chromosomes each.
Congenital(with birth), present at birth. The term fails to distinguish between traits which are actually inherited, and modifications acquired during prenatal life. In the interest of clear thinking its use should be avoided so far as possible.
Correlation(together relation), a relation between two variables in a certain population, such that for every variation of one, there is a corresponding variation of the other. Mathematically, two correlated variables are thus mutually dependent. But a correlation is merely a statistical description of a particular case, and in some other population the same two variables might be correlated in a different way, other influences being at work on them.
Cytology(cell word), the study of the cell, the constituent unit of organisms.
Determiner(completely end), an element or condition in a germ-cell, supposed to be essential to the development of a particular quality, feature, or manner of reaction of the organism which arises from that germ-cell. The word is gradually falling into disuse, and "factor" taking its place.
Dominance(mastery), in Mendelian hybrids the capacity of a character which is derived from only one of two generating gametes to develop to an extent nearly or quite equal to that exhibited by an individual which has derived the same character from both of the generating gametes. In the absence of dominance the given character of the hybrid usually presents a "blend" or intermediate condition between the two parents.
Dysgenic(bad origin), tending to impair the racial qualities of future generations; the opposite of eugenic.
Endogamy(within mating), a custom of some primitive peoples, in compliance with which a man must choose his wife from his own group (clan, gens, tribe, etc.).
Eugenic(good origin), tending to improve the racial qualities of future generations, either physical or mental.
Euthenic(good thriving), tending to produce beneficial acquired characters or better conditions for people to live in, but not tending (except incidentally and indirectly) to produce people who can hand on the improvement by heredity.
Evolution(unroll),organic, the progressive change of living forms, usually associated with the development of complex from simple forms.
Exogamy(out mating), a custom of primitive peoples which requires a man to choose a wife from some other group (clan, gens, tribe, etc.) than his own.
Factor(maker), a name given to the hypotheticalsomething, the independently inheritable element in the germ-cell, whose presence is necessary to the development of a certain inherited character or characters or contributes with other factors to the development of a character. "Gene" and "determiner" are sometimes used as synonyms of factor.
Feeble-mindedness, a condition in which mental development is retarded or incomplete. It is a relative term, since an individual who would be feeble-minded in one society might be normal or even bright in another. The customary criterion is the inability of the individual, because of mental defect existing from an early age, to compete on equal terms with his normal fellows, or to manage himself or his affairs with ordinary prudence. American students usually distinguish three grades of mental defect: Idiots are those who are unable to take care of themselves, even to the extent of guarding against common physical dangers or satisfying physical needs. Their mentality does not progress beyond that of a normal two-year-old child. Imbeciles can care for themselves after a fashion, but are unable to earn their living. Their mental ages range from three to seven years, inclusive. Morons, who correspond to the common acceptation of the term feeble-minded, "can under proper direction become more or less self-supporting but they are as a rule incapable of undertaking affairs which demand judgment or involve unrestricted competition with normal individuals. Their intelligence ranges with that of normal children from seven to twelve years of age." There is necessarily a considerable borderline, but any adult whose intelligence is beyond that of the normal twelve-year-old child is usually considered to be not feeble-minded.
Gamete(mate), a mature germ-cell; in animals an ovum or spermatozoön.
Genetics(origins), for a long time meant the study of evolution by experimental breeding and was often synonomous with Mendelism. It is gradually returning to its broader, original meaning of the study of variation and heredity, that is, the origin of the individual's traits. This broader meaning is preferable.
Germinal(sprig), due to something present in the germ-cell. A trait is germinal when its basis is inherited,—as eye color,—and when it develops with nothing more than the standard environment; remaining relatively constant from one generation to another, except as influenced by reproduction.
Germ-Plasm(sprig form), mature germ-cells and the living material from which they are produced.
Hæmophilia(blood love), an inability of the blood to clot. It thus becomes impossible to stop the flow of blood from a cut, and one who has inherited hæmophilia usually dies sooner or later from hæmorrhage.
Heredity(heirship), is usually considered from the outside, when it may properly be defined as organic resemblance based on descent, or the correlation between relatives. But a better definition, based on the results of genetics, looks at it as a mechanism, not as an external appearance. From this point of view, heredity may be said to be "the persistence of certain cell-constituents (in the germ-cells) through an unending number of cell-divisions."
Heterozygote(different yolk), a zygotic individual which contains both members of an allelomorphic pair.
Homozygote(same yolk), an individual which contains only one member of an allelomorphic pair, but contains that in duplicate, having received it from both parents. A homozygous individual, having been formed by the union of like gametes, in turn regularly produces gametes of only one kind with respect to any given factor, thus giving rise to offspring which are, in this regard, like the parents; in other words, homozygotes regularly "breed true." An individual may be a homozygote with respect to one factor and a heterozygote with respect to another.
Hormones(chain), the secretions of various internal glands, which are carried in the blood and have an important specific influence on the growth and functioning of various parts of the body. Their exact nature is not yet understood.
Inbornusually means germinal, as applied to a trait, and it is so used in this book. Strictly speaking, however, any trait which appears in a child at birth might be called inborn, and some writers, particularly medical men, thus refer to traits acquired in prenatal life. Because of this ambiguity the word should be carefully defined when used, or avoided.
Inherent(in stick), as used in this book, is synonymous with germinal.
Induction(in lead), a change brought about in the germ-plasm with the effect of temporarily modifying the characters of an individual produced from that germ-plasm; but not of changing in a definite and permanent way any such germ-plasm and therefore any individual inherited traits.
Innate(inborn), synonymous with inborn.
Latent(lie hidden), a term applied to traits or characters whose factors exist in the germ-plasm of an individual, but which are not visible in his body.
Law, in natural science means a concise and comprehensive description of an observed uniform sequence of events. It is thus quite different from the law of jurists, who mean a rule laid down for the guidance of an intelligent being, by an intelligent being having power over him.
Mendelism, a collection of laws of heredity (see Appendix D) so-called after the discoverer of the first of them to become known; also the analytical study of heredity with a view to learning the constitution of the germ-cells of animals and plants.
Mendelize, to follow Mendel's laws of inheritance.
Mores(customs), the approved customs or unwritten laws of a people; the conventions of society; popular usage or folk-ways which are reputable.
Mutation(change), has now two accepted meanings: (1) a profound change in the germ-plasm of an organism such as will produce numerous changes in its progeny; and (2) a discontinuous heritable change in a Mendelian factor. It is used in the first sense by De Vries and other "mutationists" and in the second sense by Morgan and other Mendelists; confusion has arisen from failure to note the difference in usage.
Normal Curve, the curve of distribution of variations of something whose variations are due to a multiplicity of causes acting nearly equally in both directions. It is characterized by having more individuals of a mediocre degree and progressively fewer above and below this mode.
Nucleus(little nut), a central, highly-organized part of every living cell, which seems to play a directive rôle in cell-development and contains, among other things, the chromosomes.
Patent(lie open), a term applied to traits which are manifestly represented in the body as well as the germ-plasm of an individual. The converse of "latent."
Probability Curve, the same as normal curve. Also called a Gaussian curve.
Protoplasm(first form), "the physical basis of life"; a chemical compound or probably an emulsion of numerous compounds. It contains proteins which differ slightly in many species of organism. It contains carbon, hydrogen, oxygen, nitrogen, sulphur, and various salts, but is so complex as to defy exhaustive analysis.
Psychiatry(soul healing), the study of diseases of the mind.
Recessive(draw back), the converse of dominant; applied to one of a pair of contrasted Mendelian characters which can not appear in the presence of the other.
Regression(back go), the average variation of one variable for a unit variation of a correlated variable.
Segregation(aside flock), (1) as used in eugenics means the policy of isolating feeble-minded and other anti-social individuals from the normal population into institutions, colonies, etc., where the two sexes are kept apart. (2) The term is also used technically in genetics, to refer to the discontinuity of the variation of characteristics resulting from the independent distribution of factors before or at the time of formation of the gametes.
Selection(apart pick), the choice (for perpetuation by reproduction) from a mixed population, of the individuals possessing in common a certain character or a certain degree of some character. Two kinds of selection may be distinguished: (1) natural selection, in which choice is made automatically by the failure to reproduce (through death or some other cause) of the individuals who are not "fit" to pass the tests of the environment (vitality, disease resistance, speed, success in mating, or what not); and (2) artificial selection, in which the choice is made consciously by man, as a livestock breeder.
Sex-Limited, a term applied to traits which differ in the two sexes, because influenced by the hormones of the reproductive glands. Example, the beard.
Sex-Linked, a term applied to traits which are connected with sexaccidentallyand not physiologically in development. The current explanation is that such traits happen to be in the same chromosome as the determiner of maleness or femaleness, as the case may be. Color-blindness is the classical example in man.
Sexual Selection, the conscious or unconscious preference by individuals of one sex, or by that sex as a whole, for individuals of the other sex who possess some particular attribute or attributes in a degree above or below the average of their sex. If the deviation of the chosen character is in the same direction (plus or minus) as inthe chooser, the mating is called assortative; if in one direction independent of the characteristic of the chooser, it is called preferential.
Soma(body), the body as distinguished from the germ-plasm. From this point of view every individual consists of only two parts,—germ-plasm and soma or somatoplasm.
Trait, a term used by geneticists as a synonym of "character."
Unit-Character, in Mendelian heredity a character or alternative difference of any kind, which is apparently not capable of subdivision in heredity, but is inherited as a whole, and which is capable of becoming associated in new combinations with other characters. The term is now going out of use, as it makes for clearer thinking about heredity to fix the attention on the factors of the germ-cells instead of on the characters of the adult.
Variation, a deviation in the size, shape, or other feature of a character or trait, from the mean or average of that character in the species.
Vestigial(footstep), a term applied to a character which at some time in the evolutionary history of the species possessed importance, or functioned fully, but which has now lost its importance or its original use, so that it remains a mere souvenir of the past, in a degenerated condition. Example, the muscles which move a man's ears.
Zygote(yolk), the fertilized egg-cell; the united cell formed by the union of the ovum and spermatozoön after fertilization.
Zymotic, caused by a microörganism,—a term applied to diseases. Example, tuberculosis.