FIG. 1. Diagram showing the present theory as to the adaptive radiation of the Proboscidea. June, 1921. IV. 8. Brevirostrinæ, short-jawed bunomastodonts, which imitate both the true mastodonts and the elephants in the abbreviation of the lower jaw and the early loss of the inferior tusks. These animals wandered all over Europe, Asia, and western North America. ELEPHANTOIDEA (the Elephant stock) 9. Stegodontinæ, the original members of which were doubtless ancestral to all the higher elephants, persist as an independent branch into the Lower Pleistocene of eastern Asia. 10. Loxodontinæ, embracing the great African division of the elephants beginning with Loxodonta antiqua of the Upper Pliocene, which wandered all over southern Eurasia and radiated widely over Africa. 11. Mammontinæ, including (a) the Southern Mammoths (Elephas planifrons of India and E. Meridionalis of Europe), from which is derived E. imperator of North America, and (b) the Northern Mammoths, which probably include E. columbi and the widespread E. primigenius of the northern steppes; tetradactyl pes. 12. Elephantine, the true elephants (E. indicus of India), which do not appear until the Upper Pleistocene; pentadactyl pes. This twelve-fold branching of the proboscideans is similar to the adaptive radiation which the author has traced in the evolution of the horses, of the rhinoceroses, and of the titanotheres, carrying the fundamental lines of separation back to the Middle Miocene as the most recent date, and to the Middle or Lower Eocene as the most remote date. It will be observed from the diagram that the shaded areas represent those phyla of which remains have been discovered. The large unshaded area includes the entire Oligocene, Miocenes, and Lower and Middle Pliocene history of the Elephantide which is still unknown but which is likely to be revealed at any time by discoveries both in Africa and in central Asia. A very striking fact is that the geologically earliest known member of the Elephantoidea is the Elephas planifrons of the Upper Pliocene of India, the apparent ancestor of the mammoths. 1 The first paper in this series is entitled, “A Long-jawed Mastodon Skeleton from South Dakota and Phylogeny of the Proboscidea," Bull. Geol. Soc. Amer., March, 1918; the second paper, "Evolution, Phylogeny, and Classification of the Proboscidea," Amer. Mus. Novitates, No. 1, January 31, 1921 (Osborn, 1921. 514); the third paper, "First Appearance of the True Mastodon in America," Amer. Mus. Novitates, No. 10, June 15, 1921; the fourth paper appears in the Bulletin of the Geological Society of America, under the title, "Evolution, Phylogeny, and Classification of the Mastodontoidea;" the present is the fifth paper. The Iconographic Type Revision will form one of the Memoirs of the American Museum of Natural History. Herluf Winge, 1906, p. 172. • Ibid. It is a question whether this subfamily is nearest the Mastodontidæ, with which its members are generally placed by European palæontologists. A CASE OF REARRANGEMENT OF GENES IN DROSOPHILA1 BY A. H. STURTEVANT COLUMBIA UNIVERSITY, NEW YORK CITY Communicated by T. H. Morgan, May 28, 1921 Seven mutant genes of Drosophila simulans have been shown to be allelomorphic to previously known mutant genes of D. melanogaster.2 Five of these lie in the X-chromosome, and a study of their linkage relations was shown to indicate that the sequence of the five loci concerned is the same in both species, and that the percentages of crossing over in comparable regions, while not indentical, is still not very different. The other two allelomorphic mutant genes, scarlet and peach, lie about 3 units apart in the third chromosome of melanogaster; in simulans they lie in the same chromosome (which is thus identified as the third one), but they were found to be at least 45 units apart. More recently two more mutant genes of simulans that lie in the third chromosome have been studied. One of these, dachs,3 lies to the left of scarlet; the other, deltoid, lies between scarlet and peach. The latter, since it makes possible the detection of a portion of the double crossovers, has resulted in a more accurate determination of the scarlet peach distance. The map based on the linkage relations of these four loci (not corrected for unobservable double crossing over) is shown in figure 1. Both of the new mutant types, dachs and deltoid, resemble previously known mutant types in D. melanogaster. Dachs in melanogaster lies in the second chromosome; it is accordingly not surprising that tests have shown it not to be allelomorphic to dachs simulans. Deltoid resembles delta melanogaster. Since both genes are dominant, the usual test of allelomorphism could not be applied; but each has also a recessive lethal effect, and crosses of delta melanogaster by deltoid simulans have shown that the hybrids that receive both mutant genes do not develop. It follows that the two genes are allelomorphic. The map of the melanogaster third chromosome, including the known ends and the three loci occupied by parallel mutations, is shown in figure 2.5 A comparison of figures 1 and 2 shows that the three identical loci are not in the same sequence in the two species. Mr. D. E. Lancefield has obtained evidence suggesting a similar rearrangement of genes in the X-chromosome of D. obscura. His results were obtained before those here reported, but are not yet published. Since D. obscura has not yet been crossed with any other species, the evidence for identity of loci is not conclusive in this case. The only analogous case so far reported appears to be that briefly described by Bridges under the name of "vermilion duplication." In this case a section from near the middle of an X-chromosome of D. melanogaster appears to have broken loose and attached itself to the left-hand end of a normal X-chromosome. Weinstein' has shown that such an occurrence might lead to a change of sequence of identical loci such as is here reported. If we suppose that the simulans third chromosome was originally constituted as is that of melanogaster, the situation as we now find it may be supposed to have arisen as follows: A section, including the peach locus, broke loose and attached itself near the right-hand end of a normal third chromosome. After this condition had become established the peach locus near the middle of the chromosome mutated or became "deficient," so that in effect the peach locus was moved to the right end of the chromosome. Such an interpretation will account for the observed facts.8 There is, however, another possible method whereby the same result might be supposed to have been brought about, viz., by the simple inversion of a section of a normal chromosome. Such an accident seems not unlikely to occur at the stage of crossing over. If we suppose a chromosome to occasionally have a "buckle" at a crossing over point, it is conceivable that crossing over might be followed by fusion of the broken ends in such a way as to bring about an inversion of a section of chromosome. Either of the two suppositions discussed will account for the observed results, but they should lead to different relations for other loci in the same chromosome; it is hoped that further work will lead to the discovery of additional parallel mutations, so that the maps may be studied in more detail. If an inversion of the kind suggested above occurred within a species, then individuals bearing one normal chromosome and one chromosome with an inverted section would probably show no crossing over in the region in question, since it seems probable that synapsis in this region would be abnormal or absent. It would also be not surprising if crossing over in adjacent regions was decreased. But individuals homozygous for the inverted section would be expected to show free crossing over again, since there should now be no difficulty at synapsis. The relations indicated are those that have actually been found in the cases of the two "crossover genes" in melanogaster known as C and CII.10 These "genes" both cause, in individuals heterozygous for them, the disappearance of crossing over in the immediate regions where the "genes" themselves lie, and a considerable reduction of crossing over in neighboring regions. In individuals homozygous for either of these "genes," however, the percentage of crossing over rises to (or beyond) that found in "normal" individuals. Experiments are now under way in an attempt to determine if these "genes" are really simply inverted chromosome sections, but it will probably be a long task to definitely settle the matter. The demonstration of a change in sequence of identical loci that is here reported makes the identification of parallel mutations in species that cannot be crossed even more difficult than it has previously seemed; for identity of sequence in a group of identical loci now appears not to be necessarily expected. 1 Contribution from the Carnegie Institution of Washington. 2 Sturtevant, A. H., Genetics, 6, 1921 (63, 179). 3 Discovered by Prof. T. H. Morgan. Discovered by Dr. C. B. Bridges. This map is based on the more extensive one published by Bridges in these PRO CEEDINGS. 6 Bridges, C. B., J. Gen. Physiol., 1, 1919 (645). 7 Weinstein, A., these PROCEEDINGS, 6, 1921 (625). 8 It is, of course, possible to invert this interpretation by supposing the simulans situation to be the original one. 9 Muller, H. J., Amer. Nat., 50, 1916 (103, 284, 350, 421), and Sturtevant, A. H., Carnegie Inst. Wash. Publ., No. 278, 1919 (305). 10 Sturtevant, A. H., these PROCEEDINGS, 3, 1917 (555), and Carnegie Inst. Wash. Publ., No. 278, 1919 (305). A REMEASUREMENT OF THE RADIATION CONSTANT, h, BY MEANS OF X-RAYS BY WILLIAM DUANE, H. H. Palmer AND CHI-SUN Yeh JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY Communicated July 6, 1921 Since the discovery of the fact that the continuous X-ray spectrum has a short wave-length limit, which obeys the quantum law, a number of experimentors have used this phenomenon to determine the value of h.2 In its application to X-rays the quantum law may be expressed by the equation Ve = hv, (1) where V represents the maximum difference of potential in the X-ray tube through which the electrons fall, e, the charge carried by each electron, v, the frequency of vibration corresponding to the short wave-length limit of the spectrum, and h, Planck's action constant. Evidently a measurement of V and v gives us the ratio of h to e, and from this we get h, if we suppose e to be given by other experiments. Blake and Duane3 |