August 24, 1905] 



NA TURE 



413 



Perhaps when we can ascertain the- temperature at which 

 silicate magmas pass from the metastable to the labile con- 

 dition we may use this knowledge to determine the exact 

 temperature at which certain of their minerals crystallised. 



Ordinary petrographical descriptions supply numerous ex- 

 amples of the difference between the metastable and labile 

 conditions to anyone who will read them in the light of 

 the suggestion which I have made; others are to be found 

 in such experiments as those of \'ogt or Doelter. 



My own hope is that when more experiments have been 

 made upon mixed supersaturated solutions it will be found 

 that most, if not all, of the features of rock development 

 are paralleled by the ordinary processes of crystallisation, 

 but that motion, supersaturation, and supercooling are most 

 important factors. 



The very similarity between the differentiation of the 

 alumo-alkaline and ferro-magnesian minerals on a small 

 scale in the rock, and that of the alumo-alkaline (or salic) 

 and ferro-magnesian (or femic) rocks themselves on a large 

 scale, points to some similarity of origin. 



In order to avoid burdening this Address with detail I 

 have merely chosen the researches of van 't Hoff, Vogt, 

 Doelter, and Heycock and Neville as illustrations of ex- 

 perimental work conducted on the lines of modern physical 

 chemistry, and have omitted much that might have been 

 mentioned ; the valuable researches of Pelouze, Lagorio, 

 Morozewicz, and Loevinson-Lessing, and the melting-point 

 determinations of Joly I have not quoted, because they 

 belong for the most part to an earlier period than that 

 which I am considering, and have been discussed by Teall 

 and other writers. 



Many very interesting speculations I have passed over 

 entirely, because my object has been to focus attention upon 

 experimental evidence. I cannot help thinking that these 

 speculations are often based upon chemical actions and 

 equilibria that may be impossible; but we cannot criticise 

 them for lack of evidence, and I return to my original 

 statement that geology is only beginning to enter the ex- 

 perimental stage. 



An earnest beginning is, however, being made. The re- 

 searches on mineral and rock synthesis which I have 

 already quoted are laying a solid foundation ; and I see no 

 reason why something of the sort which has been done 

 by van 't Hoff and his collaborators for the aqueous de- 

 posits of Stassfurt should not ultimately be worked out 

 for an igneous complex, though it may involve tenfold the 

 labour and tenfold the time. We have already to welcome 

 the establishment by the United States Geological Survey 

 of a laboratory for the express purpose of applying to 

 minerals and rocks the exact methods of modern physics 

 and physical chemistry. The very suggestive research of 

 Day and Allen upon the thermal properties of the felspars 

 is a promise of the sort of work that may be e.xpected from 

 such laboratories. 



I fear it will be only too evident to those who have 

 given me their patience during this Address that I ap- 

 proach the problems considered in it from the point of 

 view, not of the geologist or the chemist, but of the 

 crystallographer, to whom the birth and growth of crystals 

 are a study in themselves. Whether we watch with the 

 microscope a tiny crystal growing from a drop of solution, 

 or contemplate with the imagination the stages by which 

 the fiery lavas of past geological periods sank to rest and 

 crystallised, we view tlie same process ; it is the trans- 

 formation of liquid into crystal. Not necessarily into a 

 solid, for recent research shows that there is no dividing 

 line between liquid and solid ; a plastic solid body may flow ; 

 a solid glass is only a supercooled liquid ; witness, for 

 example, the experiments of Adams on rocks, and of 

 Tamman on supercooled liquids. The real primary dis- 

 tinction is between crystalline and non-crystalline material, 

 and there is even good reason to believe that some crystals 

 are liquid without ceasing to be crystals. 



The properties of most rocks,' of metals, alloys, ice, and 

 many other substances are due to the fact that they consist 

 of crystals, and the importance of the study of the latter 

 is now, I trust, being brought home alike to chemists, 

 physicists, geologists, and engineers in connection with 

 problems relating to the strength, the movements, the 

 origin and changes of what are usually called solids. 



And so I close, as befits a student and teacher of crystal- 

 lography, with the hope that renewed attention iray be 

 paid to this subject, and that it may attract the interest 

 of many a keen intellect in South Africa. The higher 

 scientific studies are now establishing themselves as an in- 

 tegral part of the educational and intellectual life of the 

 country : this is in no small measure due to the South 

 African Association ; and we may hope that the visit of 

 the British Association will be of some help to her younger 

 sister in the task of diffusing a taste and an interest for 

 the pure truths of science and the studies that they both 

 hold dear. 



SECTION D. 



ZOOLOGY. 



Opening Address by G. A. Boulenger, F.R.S., V.P.Z.S., 



President of the Section." 



The Distribution of African Fresh-water Fishes. 



I think I may ascribe the honour of having been chosen 

 to preside over this Section to the fact that I have specially 

 applied myself to the study of a large class of the animals 

 of the part of the world in which we are for the first 

 time assembled. The subject of the Address which it is 

 the custom to deliver on such an occasion was therefore 

 not difficult to choose — a general survey of the African 

 fresh-water fishes from the point of view of their distri- 

 bution. 



It has repeatedly been pointed out that no division of 

 the world can answer for all groups of animals, differences 

 due to the period at which they appeared and to their 

 ability or inability to spread over obstacles, whether of 

 land or water, precluding any attempt to make their pre- 

 sent distribution fit into the frames of the general zoo- 

 geographer. The great divisions of the earth, as outlined 

 by our eminent Vice-President, Dr. Sclater, nearly half a 

 century ago, and based mainly on a study of passerine 

 birds, have therefore varied considerably according to the 

 standpoint of the many workers who have followed in his 

 footsteps. Fresh-water bony fishes particularly lend them- 

 selves to a uniform treatment, their principal groups 

 having sprung up, so far as palajontological data teach 

 us, about the same period in the history of the earth, and 

 branched off in many directions within a geologically 

 speaking brief lapse of time, most of them, besides, being 

 regulated in their distribution by the water-systems. How 

 greatly their distribution differs from that of terrestrial 

 animals has long ago been emphasised. Thus, latitudinal 

 range, so striking in many African reptiles, does not exist 

 in fishes : the key to their mode of dispersal is, with few 

 exceptions, to be found in the hydrography of the con- 

 tinent ; and, as first shown by Dr. Sauvage, latitude and 

 climate, excepting of course very great altitudes, are in- 

 considerable factors, the fish-fauna of a country deriving 

 its character from the head waters of the river-system 

 which flows through it. In this way, for instance, the 

 Lower Nile is inhabited by fishes bearing a close re- 

 semblance to, or even specifically identical with, those of 

 Tropical Africa, and strikingly contrasting in character 

 with the land-fauna on its banks. Such being the case, it 

 seems at first as if the geographical divisions of the fish- 

 fauna were a matter of extreme simplicity, and that a 

 knowledge of the river-systems ought to suffice for tracing 

 areas which shall express the state of things. But we 

 must bear in mind the movements which have taken place 

 on the surface of the earth, and owing to which the con- 

 ditions we find at present may not have existed within 

 comparatively recent times ; and this is where the 

 systematic study of the aquatic animals affords scope for 

 conclusions having a direct bearing on the physical geo- 

 graphv of the near past. To mention two examples, the 

 fishes of the Nile show so many specific types in common 

 with those of the Senegal-Niger, now more or less com- 

 pletely separated by the Chad basin, that we felt justified 

 in postulating a recent communication between these water- 

 systems, which has been fully confirmed by the study of 

 the Lake Chad fishes ; whilst, on the other hand, the 

 greater difference between the fishes of the Nile and those 

 of the Congo basin, the waters of which interlock at pre- 

 1 Slightly abridged. 



NO. 1869, VOL. 72] 



