DISCOVERY 



275 



A New Theory of the Origin of Mercury 



Dr. A. C. D. Crommelin has recently drawn attention 

 to a remarkable theory concerning the origin of the 

 planet Mercury, which, he holds, seems " to explain a 

 good many points that are otherwise puzzling." Ac- 

 cording to this suggestion. Mercury was originally a 

 satellite of Venus. The Earth and Venus are so near!)- 

 alike in size that it is not imlikely that they originally 

 rotated at about the same rate. The solar tides, which 

 according to Darwin's theory brought about the disrup- 

 tion of the Earth and the birth of the Moon, would be 

 still more potent on Venus, and it would not be surprising 

 if a considerably larger satellite were bom. This huge 

 satellite would yer>^ quickly recede from the planet from 

 which it was torn, and in the case of a planet re\olying 

 so near the Sun as Venus, the satellite would be likeh' 

 to escape from the control of the planet and would 

 become a planet on its owm account. Dr. Crommelin 

 remarks that the long rotation period of Mercury tends 

 to support this tentative hypothesis, and the fact that 

 the albedoes — or degrees of whiteness — of the ^loon and 

 Mercury are nearly the same suggests that they may 

 have had a similar history. The speculation is a daring 

 one, but it must be confessed that it is rather attractive. 



Professor Hale's Retirement 



There will be widespread and genuine regret all over 

 the scientific world at the announcement of the retire- 

 ment of Dr. George E. Hale from the post of director 

 of the Mount ^^'ilson Observatory on grounds of ill-health. 

 Dr. Hale will continue as honorary director, but the 

 duties of the office will be discharged by Dr. W. S. .\dams, 

 who becomes the second director of the famous institu- 

 tion. Dr. Hale is a comparatively young man so far 

 as astronomers go. Born in 1868, he came into promin- 

 ence at the age of twenty-four through his invention of 

 the spectroheliograph. At the early age of twenty- 

 seven he was appointed director of the Yerkes Obser- 

 vatory, and in 1905 he was transferred to the mountain 

 observatory on Mount Wilson, with the foundation of 

 which he was closely associated. His researches at 

 Mount Wilson have been chiefly in solar astronomy, in 

 which department he has done enduring work. But no 

 less valuable has been his work in directing the activities 

 of an institution which has been largely responsible for 

 the vast widening of the scientific horizon during the 

 twentieth century. Hector Macphersox. 



Reviews of Books 



PLANT-LIFE IN THE DISTANT PAST 



Studies in Fossil Botany. By Dukinfield H. Scott, 



LL.D., F.R.S., etc. Vol.' II, Spermophyta. Third 



Edition. (21s.) 



This volume, which deals with seed-bearing plants, 



has been in part rewTitten ; knowledge has advanced 



and \dews have changed. It is mainly concerned with 



the higher types of vegetation preserved in association 



with our coal seams, and the author contents himself 

 with a comparatively short account of some of the more 

 important genera discovered in strata younger than those 

 of the Paleozoic era. The flowering plants, the dominant 

 class of Spermophj-ta at the present time, do not come 

 within the scope of the book. This selective method of 

 treatment has its advantages : the student is intro- 

 duced to a phase of evolution about which we have 

 acquired in recent years abundant information ; he is not 

 b^wildered by descriptions of obscure, problematical 

 fossils which tell us little that is botanically interesting. 



Dr. Scott's researches into the structure of Palaeozoic 

 plants ha\-e not only tltfown a flood of light upon the 

 wonderful vegetation of the remote past, but they have 

 largely contributed to the recognition of the fact that 

 students of botany cannot afford to neglect types that 

 have long ceased to exist if they are interested in the 

 problems of evolution. His work is conspicuously 

 ■scientific in the best sense ; he combines sound scholar- 

 ship with caution, and he has the ability to make a difficult 

 subject clear to readers whose know^ledge of botany is 

 comparatively slight. The book is hardly an elementary 

 treatise ; it is an admirable, critical review of the present 

 state of our knowledge of those sections of the plant 

 kingdom chosen for special treatment. A particularly 

 valuable chapter is that on General Results, in w-hich 

 the author summarises all the available information on 

 the older \ascular plants. The more we know of the 

 floras of the past, the more difficult becomes the problem 

 of evolution. A superficial acquaintance w-ith extinct 

 types of vegetation may lead to the conclusion that the 

 records of the rocks clearly support the orthodox con- 

 ception of a progressive development from the simple 

 to the complex, but when we become familiar with the 

 extraordinary complexity and astounding variety of 

 types illustrated by the plants which formed the forests 

 of the Coal Period, and contrast them with their nearest 

 li\dng relatives, we wonder more and inore whether it 

 will ever be possible to construct a satisfactory liistory ot 

 the vegetable kingdom from the fragmentary documents 

 within our reach. " In our complete ignorance, now 

 reaUsed, of the methods of Evolution, we cannot hope 

 for very definite success in tracing its course. A more 

 tentative and diffident tone seems to be demanded in 

 discussing phylogenetic problems, and may be found, it 

 is hoped, in the present issue of this book." 



The second volume, like the first, is well illustrated 

 and pro\ided with adequate references to literature. 

 The types dealt with in the " Studies " are not merely 

 interesting as examples of structiu-al complexity and of 

 the high stage of development represented by members 

 of the older floras ; they Eire striking illustrations of the 

 uniformity through the ages of the main features of 

 plant-mechanism revealed by the almost perfect sections 

 of petrified stems, leaves, and seeds preserved in the later 

 Palaeozoic rocks. We can not only reconstruct the 

 framework of the plant-machine, but we can use our 

 knowledge of the physiology of modern plants as a guide 

 to the conditions under which the vegetation of remote 

 ages had its being. 



