June 12, 1913] 



NATURE 



387 



for a number of aquatic forms of life, chiefly the 

 fresh-water mussels, the Pleurocerida;, and the cray- 

 fishes, covering the region from the New York- 

 Pennsylvania State line to the northern boundary ot 

 l'ennessee. The main results are that the groups 

 mentioned have not been transported overland to any 

 extent, and consequently are apt to furnish evidence 

 as to the former drainage conditions. The Allegheny 

 Mountains have acted most of the time as an etlective 

 barrier to the dispersal of fresh-water life, at least 

 since the end of the Cretaceous. The Atlantic side 

 received its fauna from the interior basin — not 

 across the mountains, but around the northern and 

 southern ends. A few instances are known where 

 single species crossed the divide, and these cases are 

 found in two sharply restricted regions. 



Progressive evolution among hybrids of Oenothera 

 was discussed by Prof. Bradley M. Davis, of the 

 University of Pennsylvania. Certain cultures of 

 hybrids between Oenothera biennis and O. grandi- 

 f.ora have presented in the second generation a high 

 degree of progressive advance in flower size and in 

 the size of the leaves and the extent of their crinkling. 

 An hypothesis for such progressive evolution is offered 

 by the Mendelian principle of recombination of factors 

 for large size on the assumption of multiple factors 

 for the dimensions of organs, but this hypothesis also 

 demands the presence in the same culture of groups 

 of plants containing the factors for small size. When 

 in an F 2 generation there is a considerable group ol 

 plants with flowers larger than those of the larger 

 parent there should also be expected corresponding 

 groups with flowers as small as, or smaller than, those 

 of the smaller parent. In F 2 generations of about 

 1000 and 1500 plants respectively there were no groups 

 of plants with flowers as small as, or smaller than, 

 those of O. biennis, the small-flowered parent. The 

 cultures as a whole presented a marked advance in 

 flower size. A similar situation was presented by the 

 character of the foliage in certain F„ generations 

 The leaves throughout the mass of these cultures were 

 much larger than those of the parents and generally 

 much more crinkled. It is difficult to explain the 

 results on strict Mendelian principles of segregation. 

 Admitting- the complexity of the situation when such 

 an extreme cross is made as that between O. biennis 

 and O. grandiflora, there still appears to Dr. Davis 

 sufficient reason in the data at hand to present the 

 problems as material for reflection on the Mendelian 

 theory of the stability of factors and the principles of 

 their distribution unchanged in the organisation of 

 gametes. 



Attention was given to the subdivision of the United 

 States into climatic areas more or less susceptible of 

 quantitative definition by Prof. Burton E. Livingston, 

 professor of plant physiology in Johns Hopkins Uni- 

 versity, in a paper on climatic areas of the United 

 States as related to plant growth. From a thorough 

 study of the climatic data which are at hand, it 

 appears that any two systems of isoclimatic lines, one 

 system representing the geographical distribution of 

 temperature conditions and the other representing that 

 of moisture conditions, have a strong tendency to 

 cross each other, thus dividing the country into 

 climatic areas, each one capable of quantitative de- 

 scription. 



Dr. William Duane, late of the Curie Radium 

 Laboratory, University of Paris, referred to some un- 

 solved problems in radio-activity (illustrated). He 

 discussed such questions as : How can atoms which 

 are physically and chemically similar to each other yet 

 be so different that some of them will disappear imme- 

 diatelv and others not for a long time? The explana- 

 tion of this probably lies in the internal structure of 



NO. 2276, VOL. 91] 



the atom and not in external causes, for external 

 conditions have no known effect upon the phenomenon. 

 The second unsolved problem to which attention was 

 directed was connected with the rays given off by the 

 substances during their transformations. The third 

 problem had to do with the y rays. He asked : Is the 

 y ray a wave form spreading out as sound waves do 

 from their source, or is it of corpuscular nature re- 

 sembling the sparks projected from an exploding 

 rocket? The fact that the /3 ray, which the y ray 

 is capable of producing, does not depend upon the 

 distance from the source of the y ray to the point at 

 which the P ray is produced seems to indicate that the 

 latter hypothesis is correct. 



Dr. Edward C. Pickering, director of the Harvard 

 College Observatory, introduced the subject of the 

 determination of visual stellar magnitudes by photo- 

 graphy. Ordinary photographic plates are most sensi- 

 tive to blue light, while the yellow rays are those that 

 effect the eye most strongly. Accordingly, blue stars 

 appear brighter and red stars fainter in a photograph 

 than to the eye. Isochromatic plates are, however, 

 manufactured which are very sensitive to yellow light. 

 If a yellow screen is interposed, the blue light is cut 

 off and red stars appear even brighter, relatively, than 

 they do to the eye. By using a thin yellow screen 

 which cuts off only a portion of the blue rays it is 

 possible to obtain plates having the same colour index 

 as the eye. To fulfil this condition several blue and 

 several red stars have been selected near the north 

 pole. Photographs are then taken with different 

 screens until one is found which gives images of the 

 same relative brightness as the naked eye. With the 

 16-in. Metcalf telescope at Harvard, stars as faint as 

 the twelfth magnitude may be photographed in this 

 way with an exposure of ten minutes. With an expo- 

 sure of two hours, stars can be photographed about 

 as faint as they can be seen with a telescope of the 

 same size. On a perfectly clear night a photograph 

 is taken of the north pole with exactly ten minutes' 

 exposure, then similar exposures on four different 

 regions, then a second time on the north pole, on five 

 other regions, and a third time on the north pole. The 

 twelve plates are developed together and various pre- 

 cautions taken to secure uniform results. The mag- 

 nitudes of numerous stars near the north pole have 

 been measured with great care, and the magnitudes 

 of the stars on the other plates can thus be determined 

 on the same scale. 



The spectroscopic detection of the rotation period of 

 Uranus was the subject of a paper by Dr. Percival 

 Lowell and Dr. V. M. Slipher, of the Lowell Observa- 

 tory, Flagstaff, Arizona. By means of the spectro- 

 scope, it is possible to measure the speed of approach' 

 or recession of a luminous body; for the lines of the 

 spectrum are shifted toward the violet or red in pro- 

 portion as the body moves toward or from the 

 observer. Hence, if the image of a rotating planet be 

 so thrown upon the slit of the spectroscope that one 

 end of the slit is illuminated by light from the ap- 

 proaching side of the planet and the other end by 

 light from the receding side, the lines will be tilted 

 through an angle which measures the speed of rotation. 

 In this way, from spectrograms obtained at the Lowell 

 Observatorv in 1911, the authors determined the rota- 

 tion of the planet L'ranus about its axis to take 

 place in ten hours and fifty minutes, in a direction 

 opposite to that of the rotation of the planets nearer 

 the sun. Thus, for the first time, an authentic deter- 

 mination of the rotation of this planet has been made 

 by a direct method. 



' Dr. V. M. Slipher also described the spectrum of 

 the nebula in the Pleiades. Two photographs of _ the 

 spectrum of the faint nebula near Merope, a bright 



