The law of filial regression was based upon a statistical study of many facts of observation. But the principles of Mendelian inheritance preclude our adopting the interpretation of it given by Galton. An albino with pigmented ancestors does not revert toward the average color of its species. Extreme forms are often extreme because they represent an exceptional degree of fluctuating or, somatic variability combined with a marked germinal variation in the same direction. As the somatic modifications are not transmitted to a marked extent, if at all, the offspring would probably exhibit a somatic character not far from the average that would be expected from its hereditary composition. Hence there would be more or less reversion toward the general mean.

REFERENCES

BABCOCK, E. B., and Clausen, R. E., Genetics in Relation to Arigiculture. N. Y., McGraw-Hill, 1918.

BATESON, W., Mendel's Principles of Heredity. Cambridge University Press, 1909.

CASTLE, W. E., Genetics and Eugenics (3d. ed.). Harvard University Press, 1924.

DONCASTER, L., The Determination of Sex. Cambridge University Press, 1914.

EAST, E. M., AND JONES, D. F., Inbreeding and Outbreeding. Philadelphia, Lippincott, 1919.

GALTON, F., Natural Inheritance. London, Macmillan, 1889.

GOLDSCHMIDT, R., The Mechanism and Physiology of Sex Determination. London, Methuen, 1923.

MORGAN, T. H., Heredity and Sex. Columbia University Press, 1913. The Physical Basis of Heredity. Phila., Lippincott, 1925. STURTEVANT, A. H., MULLER, H. J., AND BRIDGES, C. B., The Mechanism of Mendelian Heredity. N. Y., Holt, 1923. PUNNETT, R. C., Mendelism (6th ed.). London, Macmillan, 1919. WALTER, H. E., Genetics (2nd ed.). N. Y., Macmillan, 1922. WEISMANN, A., Essays on Heredity. Oxford, Clarendon Press, 1891-92. -The Germ Plasm. London, Scott, 1893.

VIGGAM, A. E., The Fruit of the Family Tree. Indianapolis, BobbsMerril24.

CHAPTER XV

HEREDITY AND ENVIRONMENT

A perennial and much discussed question concerning heredity is whether or not characters acquired by an organism may be passed on to its descendants. Organisms are continually acquiring characteristics of one kind or another as a result of their own activities or the influence of environmental changes. If we expose ourselves to the sun we are apt to acquire sunburn or freckles, and if we engage in severe manual labor the skin of our hands becomes thickened, or calloused. These modifications are typical illustrations of acquired characters as contrasted with germinal characters, such as blue or brown eyes, which depend upon the constitution of the germ plasm. It is well known that germinal variations, like albinism or supernumerary fingers, may be transmitted to subsequent generations. A few decades ago it was generally held that acquired characters were also transmitted. Usually, in fact, no distinction was made between germinal and somatic modifications. There seemed to be no reason why any kind of character might not be inherited as well as any other.

The doctrine of the transmission of acquired characters was brought into especial prominence by the great French naturalist Lamarck, who was one of the most distinguished of the pre-Darwinian proponents of the theory of organic evolution. According to Lamarck the transmission of acquired characters is the chief cause of evolutionary changes. It must be conceded that there are many features of structure and behavior which may be very plausibly explained if acquired modifications are passed on by inheritance. The soles of our feet, for instance, are relatively thicker than the rest of the skin and their increased thickness is manifested even in the embryo. Since walking

upon the ground causes the soles of the feet to become thicker we need only assume that this modification is hereditary in order to account for the fact that we are already equipped at birth with thickened soles. The Lamarckian theory seems also to afford a reasonable explanation of rudimentary organs. Inasmuch as the disuse of organs frequently causes them to diminish in size, the cumulative hereditary effects of disuse would, in time, lead the organs to become rudimentary and eventually to disappear. The numerous blind spe

cies of cave animals with rudimentary eyes are often cited as evidence for the inherited effect of disuse.

FIG. 175-Lamarck.

The inheritance of acquired characters, or the Lamarckian factor, as it is often called, was accepted by Charles Darwin and most of the earlier advocates of the theory of evolution. Lamarck had no theory as to why acquired characteristics come to be inherited, but Darwin was strongly impressed with the importance of gaining some insight into the causal mechanism of heredity and variation, and, as a step in that direction, he propounded his provisional theory of pangenesis. As Darwin was something of a Lamarckian as well as a Darwinian, he naturally held that one of the requirements of an adequate theory of heredity was that it should give a plausible explanation of the transmission of acquired characters, and his theory seems especially designed for this purpose. The fundamental assumption is that the various cells of the body give off into the blood or other fluids very minute living units called gemmules, and that the gemmules come to be stored in the germ cells, which are supposed to have a special affinity for attracting these bodies. When the germ cells give rise to a new individual, the gemmules,

which are assumed to be of many different varieties, are supposed to produce each its own kind of organ. Each part of the offspring was thus held to be derived from the corresponding part of the bodies of the parents. Should an organ be increased in size through frequent use, it would, according to Darwin, produce more gemmules. More gemmules would therefore be stored in the germ cells, and hence the resulting individual would have the corresponding organ more fully developed.

Granted the premises of the theory, the transmission of acquired characters is readily accounted for. The theory is

g

FIG. 176-Schematic representation of theories of heredity. The upper diagrams illustrate Darwin's theory of pangenesis. s, various kinds of body cells from which gemmules are given off which collect in the germ cells, g. The latter give rise to successive generations and their component body cells as indicated by the arrows. The lower series illustrates Weismann's conception of the continuity of the germ plasm and the derivation of the body cells, s, from the germ cells, g.

ingenious, though highly speculative, and it has the merit of bringing together many facts of heredity and variation under a common point of view. Nevertheless, it failed to secure many adherents. The fundamental assumptions of the theory are inconsistent with the facts of physiology, and direct evidence for them is entirely lacking. Moreover, much has been learned

in regard to the probable basis of hereditary transmission, and this knowledge has given us a picture of the process very different from the one presented by the theory of pangenesis.

One of the first to question the Lamarckian theory was Francis Galton, but skepticism on the subject did not become general among biologists until after the theory was attacked by Professor August Weismann. In his early essays on heredity, Weismann developed a theory of transmission based on the idea of the continuity of the germ plasm. The germ plasm, or material basis of heredity, is, according to Weismann, distinct from the body plasm, although it may produce the latter in the course of embryonic development. Germ plasm, strictly speaking, is not derived from the body, although it is carried and nourished by the body. Its source is in the germ plasm of preceding generations. Germ plasm is supposed to be handed on from one generation to the next by a continuous series of cell generations. It is the thread that connects successive generations of individuals together, and these individuals resemble one another because they are all developed from a continuous germinal substance. Heredity, in other words, is due to the continuity of the germ plasm.

This is undoubtedly a very simple and plausible theory. Inasmuch as the germ plasm is derived in every case from antecedent germ cells instead of from the body, it would hardly be expected that characters acquired by the body would be transmitted through the germ cells to the next generation. Accordingly Weismann was led to inquire if there really is sufficient ground for the Lamarckian