October 1, 1895.] 



KNOWLEDGE. 



237 



apparently belonging to extinct species, ear-bones of whales, 

 manganese nodules, cosmic dust, and otlier inorganic 

 materials were often brought to the surface. 



We cannot hereenter into the vexed question of coral reefs. 

 Chiefly on account of observations made on board the 

 VhaUengcr, a general feeling arose that Darwin's " sub- 

 sidence theory " was inadequate. In 1880, Dr. Murray 

 propounded another explanation, maintaining that coral 

 reefs have grown up from the tops of submerged and half 

 submerged banks and mountains, and between the two 

 hypotheses the struggle has been keen and protracted. 

 Nearly everybody admits the force of Murray's arguments, 

 but some writers stUl hold that many reefs have been 

 formed in the way suggested by Darwin. 



The deep-sea temperature observations were made by the 

 naval officers on board, by means of the Miller-Casella 

 thermometer. The inquiry into the errors of these 

 instruments, made by Prof. Tait, forms in itself an 

 important contribution to our knowledge of the effects of 

 great pressures upon various substances. All the tempera- 

 ture observations have been published separately in the 

 form of curves showing the vertical distribution from 

 surface to bottom at each station, and the general result 

 was obtained that in the open sea the temperature remains 

 practically constant from year to year at all depths exceed- 

 ing one hundred fathoms. It was further found that over 

 certain large areas the temperature at the bottom was the 

 same. In the eastern part of the North Atlantic, for example, 

 the temperature at the bottom was everywhere 86-8°, while 

 in the western part of the same ocean it was 86:3°. It was 

 afterwards shown that these immense stretches of uniform 

 temperature were due to the fact that below a certain 

 depth communication with warmer or colder areas was 

 cut off by submarine ridges. 



The examination of the water samples brought home by 

 the ChaUewjer was made by the late Prof. Dittmar, who 

 confirmed the conclusion reached by Forchhammer that the 

 composition of sea water is everywhere nearly the same, 

 except that the amount of lime held in solution increases 

 at greater depths. At the same time he gave a more 

 extensive and accurate account of the proportions in which 

 the various elements are present, introducing many new 

 methods and refinements in the course of the extremely 

 difficult analyses which had to be made. 



It was thus established that the variable factors in 

 different parts of the oceans were chiefly physical, the 

 variations of temperature and density being of primary 

 importance in all questions of oceanic circulation. From 

 an enormous number of hydrometer observations made 

 during the voyage, j\Ir. Buchanan was able to construct 

 charts showing the surface density for each of the great 

 oceans, and the greatly increased data of recent years have 

 not modified the leading features of these in any important 

 respect. Mr. Buchanan also determined the salinity at 

 the depths at a sufficient number of points to give a fair 

 conception of the general distribution, although it has not 

 been possible to represent the whole by means of maps. 



Observations of oceanic currents were made on board by 

 the naval officers, but the general inquiry into the cir- 

 culation of oceanic waters was placed in the hands of 

 Dr. Alexander Buchan. The preliminary reports on the 

 sea temperature observations had settled the rival claims of 

 the gravitation and wind theories as to the cause of currents, 

 and it became evident that before the problem of oceanic 

 circulation could be profitably discussed, the facts of 

 atmospheric circulation at the earth's surface must be dealt 

 ■with. Dr. Buchan accordingly constructed a new series 

 of maps representing the distribution of atmospheric 

 pressure and temperature and the prevailing winds in 



all parts of the globe during each month of the year, 

 and, thanks to the elaborate observations made on the 

 ChalleiKjci-, he was able to extend the work over the oceans 

 with some confidence, and at the same time to contribute 

 new facts of fundamental importance to the science of 

 meteorology. This done. Dr. Buchan proceeded to the 

 oceanic problem, and his Report, an appendix to the last 

 volume, is the final publication of the series. It includes 

 a number of maps showing the distribution of temperature 

 over the oceans at vertical intervals of 100 fathoms, and 

 from these, and the known distribution of density, it 

 appears that the movements of the great masses of 

 ocean water depend directly on the influence of the great 

 atmospheric systems resting on the surface. 



Although botany was not specially represented in the 

 scientific staff' of the Challenger, very considerable collec- 

 tions were made by Mr. Moseley. These were chiefly 

 insular floras, which have been described by Mr. W. B. 

 Hemsley ; but the enormous extent of the pelagic flora 

 was first fully recognized from the observations made 

 during the cruise. 



So far as space occupied in the Reports goes, zoology 

 has by far the largest share, occupying no less than forty 

 volumes. The number of new forms discovered was 

 enormous, but it is remarkable that their description has 

 added to our knowledge of special groups rather than 

 enlightened us as to the relations existing between those 

 already known. The geographical distribution of the 

 animals obtained by the Challenger is discussed by Dr. 

 Murray in the Summary, and, taken along with the 

 further observations of more recent expeditions, the 

 results are of extreme interest and importance. The 

 observations go to show that while life is nowhere 

 absent, the animals living at the bottom in the great 

 depths are not representatives of primitive types 

 as was expected ; and it was found that at depths 

 below 1000 fathoms the number of species and genera 

 obtained in one haul of the dredge was much larger 

 in proportion to the number of individuals than is 

 the case nearer the surface. Dr. Murray believes that 

 the deeper waters were probably peopled by animals which 

 migrated from the level of the " mud-line," or zone 

 where the minute particles of organic matter derived 

 from the land are deposited on the bottom. The mud- 

 line is usually found at a depth of about 100 fathoms, 

 and the conditions in its neighbourhood are found to be 

 of great uniformity all over the world. A remarkable 

 similarity between the faunas and floras of the Arctic and 

 Antarctic regions has been traced, many species being 

 recorded in both which are not found in the intermediate 

 zones. Dr. Murray reviews the whole of the evidence 

 connected with the distribution of life, and sitggests a 

 theory which may serve to explain how the complex 

 arrangement observed was brought about. He supposes 

 that the more highly organized pelagic animals are the 

 descendants of animals which at one time inhabited the 

 region of the mud-line. The whole ocean had probably at 

 that time a uniform warm climate, and when cooling at 

 the poles began, in all likelihood in Mesozoic times, 

 animals with pelagic larvte would die out or seek warmer 

 latitudes ; hence in the tropics we find the remains of a 

 universal shallow water fauna. At the same time, the 

 lower temperature at the poles would cause descending 

 currents of colder water in those regions, and these, taking 

 a supply of oxygen downwards with them, would make 

 life for the first time possible below the mud-line, and 

 permit of gradual migrations into the deeper waters. 



We would fain linger over Dr. Murray's fascinating 

 Summary, the only part of the whole work where, in 



