In the first paper (1913) in which an attempt was made to construct a chromosome map,' the statement occurs (p. 49): "Of course there is no knowing whether or not these distances as drawn represent the actual relative spatial distances apart of the factors.... we have no means of knowing that the chromosomes are of uniform strength, and if there are strong or weak places, then that will prevent our diagram from representing actual relative distances..." Later than this3 definite evidence for the existence of genes modifying the amount of crossing over was reported, and in the "Mechanism of Mendelian Heredity" the situation was discussed as follows (pp. 67-68): "It is not supposed, however, that the per cent of crossing over represents precisely the distance between the factors, for it may be that crossing over is more likely to take place in one region than in another. In that case the distances between factors in this region calculated from the amount of crossing over between them, would be relatively greater than the actual distance.... Sturtevant has found definite factors which alter the amount of crossing over in the chromosomes, and these factors actually do affect the amount of crossing over differently in the different regions......however, ....the order of the factors remains unchanged." The three papers mentioned above were all published before Detlefsen says he began his experiments, as was also Bridges, demonstration that frequency of crossing over in the second chromosome changes with the age of the female. Since that time the question has been discussed again and again, and several cases of inherited linkage variations have been thoroughly analyzed. The conclusions to be drawn from such evidence have been well stated by Morgan,7 in a chapter devoted entirely to variationsin linkage (p. 125): "It is to be understood, then, that when we substitute the idea of distance for crossing over values the term is not used in an absolute sense, but in a relative sense, and that it depends always on the conditions of the experiment. That the genes do stand at definite levels in the chromosome, and that in this sense they are definitely spaced, seems reasonable in the light of all the evidence bearing on this point; but even if they are so spaced that crossing over is a function of their distance from each other in the series, any influence that determines how often interchange between homologous pairs will take place would give the appearance that the actual distances themselves have changed." As will be evident from the quotations given, the chromosome maps are intended to show the actual sequence of the loci, and the relative amounts of crossing over between them. The intervals between adjacent loci are not to be taken as necessarily proportional to the actual spatial distances between them-though that distance is evidently one of the elements concerned. When the amount of crossing over changes from any cause it is evident that we are dealing with a new system, and the intervals of the "normal" map will no longer be applicable (though in all cases so far investigated the sequence of loci shown in the "normal" map is unchanged). The "normal" map itself is based on the average results obtained under conditions in which no recognized disturbing factors (genetic or environmental) were known to be present. This map represents the results to be expected from untried crosses, and has shown itself to be invaluable in everyday work. Detlefsen's own account is in agreement with this, for he says that his stock of white miniature has been used in class work and has always given the value to be expected from the published maps. When values that do not agree with the "normal" map are found, analysis has always shown some disturbing factor to be present; and in all cases where the point has been investigated it has been found that maps based on data uniform with respect to this disturbing factor are entirely self-consistent. In view of these considerations it is clear that Detlefsen has misunderstood the significance of the published maps. The conception that he has attacked is one that has not been held or urged by those who have constructed chromosome maps. 1 Detlefsen, J. A., These PROCEEDINGS, 6, 1920 (663–670). 2 Sturtevant, A H., J. Exper. Zool., 14, 1913 (43-59). Sturtevant, A. H., Zs. ind. Abst. Vererb. Lehre, 13, 1015 (234–287). 4 Morgan, T. H., A. H. Sturtevant, H. J. Muller, and C. B. Bridges, New York, 1915, 262 pp. 'Bridges, C. B., J. Exper. Zoöl., 19, 1915 (1-21). Sturtevant, A. H., These PROCEEDINGS, 3, 1917 (555-558), and Carnegie Inst. Washington Publ., No. 278, 1919 (305-341); Muller, H. J., These PROCEEDINGS, 3, 1917 (619-626), and Genetics, 3, 1918 (422-499); etc. 7 Morgan, T. H., The physical basis of heredity, Philadelphia, 1919, 305 pp. ORIGIN AND HISTORY OF THE BEAR FAMILY IN THE WESTERN HEMISPHERE, WITH PARTICULAR REFERENCE TO THE RELATION OF THIS QUESTION TO PROBLEMS OF GEOGRAPHICAL HISTORY BY JOHN C. MERRIAM CARNEGIE INSTITUTION OF WASHINGTON Read before the Academy, April 26, 1921 Although bears of many varieties are widely distributed in America and have been present in large numbers for a very long period measured in terms of years, the history of this group shows that as we go back in the geological record no evidence of representatives of the bear type are present in formations of the third geological period preceding the present in America, while they are known in considerable numbers in the rocks of this age found in the Old World. There is, therefore, good reason to believe that the bear group is derived from the other side of the earth and that the ancestors of the present American bears migrated to this continent at a time geologically not far removed from the present. The bears of the world may be divided into two large groups, one some times known as the true bears, including the brown and grizzly types now widely distributed over the whole northern hemisphere. The second group is represented by the spectacle bear and is limited to the Andean region of South America. In the geological period known as the Pleistocene, immediately preceding the present, true bears like the present black bear type were associated in North America with a group of large bears known as Arctotheres, closely related to the present spectacle bear of South America. In the same period South America was inhabited by numerous Arctotheres but contained no representatives of the true bears. In the rocks of the Pliocene, or second geological period preceding the present, until recently no representatives of the bear group have been known in either North or South America. In the Pliocene deposits of Europe and Asia there are, however, remains of creatures closely related to the true bears and with these a second group originally known as the hyena bears, or the Hyaenarctos type, closely related in many characters to the Arctotheres, and through them related to the modern South American spectacle bears. Assuming that the American bears are descendants of the Old World types, it is difficult to escape the conclusion that wide land connections existed between North America and Asia, and between North and South America, at such times as to permit the migration suggested by the present distribution. As the Arctotheres are represented in the Pleistocene of South America without accompanying true bears, it is logical to assume that the Arctothere group was the first to migrate to America and that it passed through North America reaching the South American region before the true bears had spread over North America. Investigations within the last few years have shown that in the Pleistocene deposits of North America both Arctotheres and true bears are present together down to the earliest strata in which remains of bears have been discovered. There has, therefore, been reason to assume that remains of Arctothere-like bears would be found in the Pliocene of America without associated remains of the true bear type. The point of the present paper is to call attention to the fact that recent carefully conducted investigations in the Pliocence deposits in several parts of North America have brought to light remains of bear-like forms which are intermediate between the typical hyena bears of the old world Pliocene and the Arctotheres of the American Pleistocene. Such a form is represented by a specimen found in the Pliocene of Oregon. This animal corresponds very closely to the most specialized of the hyena bears of India and also approaches the Arctotheres in its structure. The evidence now available indicates that creatures of the hyena bear type came by way of broad land connections from Asia to North America in Pliocene time; that these creatures represent the type of hyena bear most nearly approaching the Arctotheres and were widely distributed in North America. There is reason to believe that from this group the Arctotheres may have developed within the American region, and that the Arctotheres by way of a wide land bridge came to people South America. The present spectacle bears of South America seem then to represent the last remnant of a group which originated in the Old World, was once widely distributed over the world, and included the largest of all known bears. AN UNIDENTIFIED BASE AMONG THE HYDROLYTIC PRODUCTS OF GELATIN DONALD D. VAN SLYKE AND ALMA HILLER Hospital of the Rockefeller Institute for Medical Research, New York, N. Y. Of the known amino acids yielded by acid hydrolysis of proteins the work of various authors' has indicated that four, viz., histidine, arginine, lysine, and cystine, are distinguished from the others by the relative insolubility of their phosphotungstates in acid solution. On the basis of this fact Van Slyke1 devised a method for separating these four amino acids as phosphotungstates, and determining them by utilizing certain characteristics of their chemical structure. The non-amino nitrogen of this group of amino acids is entirely in the histidine and arginine. The arginine was determined directly, and the histidine was estimated on the assumption that all the remaining non-amino nitrogen was in histidine. This assumption we have tested by comparing the histidine content of a number of proteins as determined in the above manner with the values determined by Koessler and Hanke's direct colorimetric method. In casein, edestin, and fibrin, the results by the two methods agree. But in gelatin the calculation based on the non-amino nitrogen indicates 6.1 per cent of the total protein nitrogen in the form of histidine, while the colorimetric method shows only 1.8 per cent. There is evidently among the products of gelatin hydrolyzed by hydrochloric acid a substance, or substances, hitherto unrecognized, precipitated with phosphotungstic acid under the conditions ordinarily utilized to precipitate the hexone bases. In attempting to isolate the substance we have precipitated it by means of phosphotungstic acid with the other bases, have redissolved the precipitate and freed it from phosphotungstic acid. The histidine and arginine were removed by precipitation with silver sulfate and barium hy droxide, and the lysine as picrate. The residual solution contained an amount of non-amino nitrogen corresponding approximately to that determined by Van Slyke's method in excess of the arginine and histidine non-amino nitrogen. Attempts to crystallize the free base or its derivatives have been successful only with the phosphotungstate. Recrystallization of the phosphotungstate yields a product in which the ratio, Total N: Amino N = 2:1. The free base prepd. from the recrystallized phosphotungstate is hygroscopic, and decomposes slowly when dried at 100°. It does not appear to be a peptide, for the ratio of amino to total nitrogen is not increased by boiling 40 hours with 20 per cent hydrochloric acid, nor by heating 8 hours in a bomb tube at 125° with 25 per cent sulfuric acid. We are engaged in the preparation of larger amounts of the substance in the hope of determining its structure. 1 Literature quoted by D. D. Van Slyke, J. Biol. Chem., 10, 1911 (15). 2 Koessler, K. K., and Hanke, M. T., J. Biol. Chem., 39, 1919 (497). GENETICAL AND CYTOLOGICAL PROOF OF NON-DISJUNCTION OF THE FOURTH CHROMOSOME OF DROSOPHILA MELANOGASTER1 BY CALVIN B. BRIDGES CARNEGIE INSTITUTION OF WASHINGTON Communicated by T. H. Morgan, March 18, 1921 A mutant type of D. melanogaster known as "Diminished" gave genetical results which proved the involvement of a "deficiency," i.e., a multilocal loss of genes, in the chromosome corresponding to the "fourth linkage group"3,4 (see section I). Further exceptions to normal inheritance showed that non-disjunction of this chromosome had occurred giving rise to (Diminished) individuals lacking one member of the fourth-chromosome pair (see section II). The deficiency in this case consisted therefore in the loss of an entire chromosome. The haploid nature of Diminished was then proved cytologically: it was found that in the cells of Diminished individuals only one small round chromosome was present instead of a pair (section III). This finding demonstrates the correctness of the view that the carrier of the genes of the fourth linkage group is the small round chromosome. A positive direct proof is provided that a particular autosome is the carrier of the genes of particular non-sex-linked Mendelian characters. I. The features that in the aggregate prove that a deficiency is involved are: |