CLINE, G. A.-Radioactivity at 25. CLINKSCALES, G. B.-The Absorp the Earth's Surface. Phys. tion of Sodium Vapor. Phys. Rev., 30. Rev., 30.

SADTPETER, J.-Radium Distribu- 29. KUEHNE, J. M.-Electrostatic Eftion. Ibid., 118.

protoplasm as the physical b life; but researches dealing w heritance and its physical ba outnumber all others concerne general biological topics. This highly significant, for it show vital the problems of heredi deemed to be at the present That this should be so is evider the fundamental importance heritance in the process of evo But biological knowledge has oped in such a way as to br gether the workers in several nally unrelated spheres of activ a combined attack upon the st resolved phenomena of inher Within the past ten years the st of experimental breeding, thos ing with organic characterist the precise methods of statisti alysis, and the investigators finer structure and detailed wo of the cell as the organic unit learned that they can no longe apart with profit. They have forces, and through their m assisted efforts, they have exp much that was previously unin ble or obscure. It is not becaus evolutionary problems are of le portance or are more fully solve heredity is now so much to the it is because the time is ripe for itable work on this particular of problems. And the results ob are valuable not only in them but also through their service in fying other subsidiary parts evolutionary process. The prese view therefore deals almost e with evolution, and primarily progress in our knowledge of her in so far as this has been d American men of science.

The Cell and Cell Physiology.-For over half a century it has been a truism that the universal "building block" of the organic world is the cell, a mass of living matter or protoplasm inclosing a nucleus. The total effects of the activities of cells, grouped to form tissues and organs and organisms, were more readily described than the physiological processes of a single unit. Perhaps the greatest task of biology has been to analyze the activities of protoplasm, and to express them in terms of known physical and chemical laws. Loeb,25 who has done so much through his own work and that of his numerous students, to make cell processes intelligible in such terms, especially as regards the stimulation of the egg to development, has added a brief paper on oxidation and its consequences. Howell 20 deals with the subject from the standpoint of the physiologist, who indeed has the best basis of attack; his paper demonstrates how complex is the chemical mechanism of the cell, involving the action of more or less obscure substances like ferments-activators, kinases, and hormones which work in the cell and throughout the entire body. Macullum 28 has directed attention to another physical process, namely surface tension, as one which plays a far greater rôle in cell physiology than has been supposed. In these papers, representative of a large class, significant evidences appear that some progress is being made -slowly, it may be, but surely-toward the final goal, which is the ultimate analysis of vital phenomena into their elements. And nothing has yet come to light to alter the general belief of biologists that these elements will prove to be the processes of physics and chemistry.

the scheme of evolution so thoroughly outlined by Darwin. So true is this that the best outline for the program of investigation and for the present review of progress is Darwin's simple statement of evolutionary method.

In the first place science must deal with the universal natural phenomenon of variation, and with its causes. The struggle for existence, consequent upon over multiplication, comes next, although this is rarely a special subject for present-day investigation. The process of selection, its reality, and its results, form the next division of the subject. Lastly heredity demands analysis, both as to the way it proceeds in the ordinary succession of unchanging generations, and as to the manner by which new characteristics may be added in evolution. Within this last field, the heredity or determination of sex has come to be so important that more than a score of papers dealing with it have appeared during the past year.

56

Variation. Although many aspects of variation are discussed in papers on other subjects, only two investigations deal with this topic primarily. Walter has studied by statistical methods the variation of Urosalpinx, a marine shore snail, using extensive representative lots taken from several localities on the east coast and from one place in California where they have been recently introduced. It appears that the snails differ somewhat in different situations, that dense populations vary less than thinly scattered communities, and that those occurring in a new locality on the west coast are in some respects more variable than a sample taken from an eastern station. The author finds, however, that a time factor brings about changes in the community characteristic during the year; his results,

Bumpus's demonstration, some years ago, that the snails of the genus Littorina had come to vary far more widely along the Atlantic Coast over which they had recently spread, than individuals taken from the original home of the species.

Evolution.- American men of science have been well to the fore in advancing our knowledge of the all-in-therefore, fall short in definiteness of clusive process of evolution. In fact, there are probably more investigators in this land now at work upon this subject than in any other country, excepting perhaps England. It is matter of considerable significance that the results now accumulating through the efforts especially of American and English men of science are in the main conformatory of and supplementary to

Another research in this field, by Harris," is a close statistical study of variation and of the correla

present questions deal largely with the chromosomes as the physical basis for the inheritance of adult characters of all kinds, including the fundamental quality of sex.

Heredity and Mendel's Principles.

tion of variations in the case of the bloodroot. His results have a high value in quantitive respects, and well illustrate the methods in this department of investigation. Selection. This is again a subject that is discussed more or less fully-On the basis of his classic experiin several papers to be considered ments, Gregor Mendel formulated two beyond. Two extremely important ar- general laws, namely: The principles ticles by Harris 18_19 prove the reof Dominance and of Segregation. ality of the selective process in na- Briefly, these are as follows: When ture in the case of a plant, Staphylea. a cross is made from two related orHe compared the fruiting organs of ganisms which differ visibly in a wide series of individuals taken at given character, the offspring resemdifferent stages of maturity from ble one parent in this character, and flowering to the formation of com- this peculiarity is said to be domplete seeds. From his exhaustive ob- inant (D) over the contrasted rejective analysis it appears that dur- cessive (R) quality which does not ing the time specified a distinct appear. This is another statement elimination occurs which bears most of what was earlier denoted "alterheavily upon individuals with certain native" inheritance as distinguished characteristics. For example, the from "blended" inheritance, where "selected" or maturing flowers have the offspring of two differing parents more ovules, they are more perfectly were intermediate. When now the symmetrical in radial respects, and hybrid organisms are bred inter se, they are more uniformly double in some of their progeny prove to posform, producing even rows of ovules. sess the dominant character only; How and why selection acts to bring others are hybrid like their parents, about these results are questions of with the recessive character subless immediate importance; the ma- merged by the dominant one, while jor fact of real qualitative selection still others are purely recessive in stands unquestioned. It is fortunate constitution and in their ability to that investigations upon natural se- transmit the R character only to lection have not been altogether done their later offspring. By a singuaway with through the operation of larly clear and prescient analysis, modern tendencies in biological study Mendel showed that the D and R to focus attention on heredity and characters entering into the first its causes. The latter subject has a hybrids were segregated during the high but by no means exclusive im- maturation of the germ-cells of the portance in evolution. hybrids, so that these gametes became pure and "free from the taint of the cross." In symbolic form the germ cells of the original parent with the dominant character would be DD; those of the other parent would be RR. The hybrids would be DR in gametic constitution. When segregation was accomplished, their germ cells would carry either D or R, and therefore the result of a cross of like hybrids would be D + RXD+R = I DD + 2 DR + I RR. In actual experiments which have been made, the numerical relations of these three classes often follow expectation with striking exactness.

Heredity in General.-Since the appearance of De Vries's Mutation Theory, now translated into English," and the rediscovery of Mendel's papers on hybridization experiments with plants, practically every investigation of biological inheritance has been undertaken to test the applicability of Mendel's principles. Hundreds of papers, and not a few textbooks like that of Bateson, have been written on this subject, and the organisms under investigation range from the lowest to the highest-from protozoa to man. Primarily as the result of the observations of cytologists on the behavior of cells during division, and especially during the maturation divisions when the germcells prepare for their later union,

The year's work has yielded important confirmation of these principles, but not without qualification in nearly all cases. The clearest re

sults have been obtained with plants. | But even in these cases some tend

Shull finds an almost typical case in Lychnis, where the purple flower color dominates over white in crosses, but he shows that the former is by no means a simple character, for it may split up so that hybrid offspring may have flowers of a diluted purple color. Again East" has shown that in cross-bred potatoes derived from plants of differing characters the results are Mendelian, with minor departures.

Davenport has published an extensive monograph on fowls, in which he presents the results of crossing such types as the silkie, the highcrested Minorca, the game, the rumpless, Houdan crested, and others. The phenomena are on the whole Mendelian, but striking departures from the expected results are sometimes found. In other papers - he points out that such departures can be understood as the products of variability in the power or potency of the physical basis or "determiner" of the usually dominant character. Pearl and Surface 40-41 also discuss heredity in hybrid poultry, on the basis of their own experiments, and their results are in essential agreement with those of Davenport. Sturtevant has investigated the inheritance of coat color in horses. discerning at least five "color factors" whose relations are Mendelian. C. and C. B. Davenport have undertaken an extensive investigation of skin pigmentation in man and its transmission in inheritance, dealing with over 300 families of lighter or darker complexions, and with mixtures of the white and negro races. Their most interesting general conclusion is that the colors of negro and white skins do not blend in mixtures, but segregate, although imperfectly. This result accords with the findings, when the children of blond and brunet parents are systematically investigated.

46

ency to segregation may be discerned in so far as the offspring tend to group themselves about some only of the intermediate points, or toward the parental extremes.

Castle discusses the results reported earlier by Miss McCracken, who held that the single-brooded and double-brooded (univoltin and bivoltin) characters manifested by silkworm moths did not behave as typical dominant and recessive qualities in the crosses. Castle, on the contrary, believes that they do. Papers of a somewhat more general nature have also been published by Spillman," and by Gates.15

66

48

syn

A monograph of surpassing interest is Tower's 55 report on experiments with beetles belonging to the genus Leptinotarsa, which includes the familiar potato bugs. He deals with crosses bred in different combination from three species, establishing as a first result that while sometimes clear Mendelian dominance and segregation occur, in other instances, where the conditions affecting the beetles during their earliest development are unusual, an ordinarily dominant character may fail to appear. Believing that "environment" had a larger influence upon the course of heredity than it had been accorded, Tower experimented G.thetically" with beetles placed in restricted areas where different forms might hybridize, under diverse natural conditions. Leaving aside the technical facts, the general conclusion is that differences in environment are potent factors for the production of specific forms. This does not mean that acquired characters are inherited, as Tower himself points out. His own words are as follows (loc. cit., p. 306): "These results indicate that in the fertilization process the two somewhat unlike germinal substances that are being combined interact one upon the other in exactly the same way that two nonliving substances would; that is, the products of the interaction are the resultant of the natures of the two substances and the conditions under which the combination took place. We must not, therefore, expect to find a factor which determines dominance, such as tem

Additional studies upon inheritance in plants, notably those of Shull, Pearl and Surface," East," and Davis,10 show that the principle of dominance does not always hold true, for hybrids may be at one point or another on the intermediate scale between the two parental extremes.