Sept. I, 1881] 



NATURE 



407 



tranger still, with biconcave vertebra", liUe tlio^e of fishes, and 

 teeth set in sockets. 



As giving, in a few word', an idea of the rapid progress in 

 this department, I may mention that Morris's " Catalogue of 

 British Fossils," published in 1S43, contained 5300 species ; 

 while that now in preparation by Mr. Etheridge enumerates 

 15,000. 



But if these figures show how rapid our recent progress ha-; 

 been, they also very forcibly illustrate the imperfection of the 

 geological record, and give us, 1 will not say a measure, but an 

 idea, of the imperfection of the geological record. The number 

 of all the described recent species is over 300,000, but certainly 

 not half are yet on our li-ts, and we may safely take the total 

 number of recent specie^ as being not le s than 700,000. But 

 in former times there have been at the very least tw elve periods, 

 in each of which by far the greater number of species were 

 distinct. True, the number of species was probably not so large 

 in the earlier periods as at present ; but if we make a liberal 

 allowance for this, we shall have a total of more than 2,oco,ooo 

 species, of which about 25,000 only are as yet upon record ; and 

 many of these are only represented by a few, some only by a 

 single specimen, or even only by a fragment. 



The progress of paleontology may also be marked by the 

 extent to which the existence of groups has been, if I may so 

 .-ay, carried back in time. Thus I believe that in 1830 the 

 eailiest known quadrupeds were small mar.upials belonging to 

 the Stonesfield slates ; the most ancient mammal now known is 

 Micfolestcs antiquiis from the Keuper of Wiirtemberg : the 

 oldest bird known in 1831 belonged to the period of the London 

 Clay, the oldest now known is the Arcbccopteryx of the Solenhofen 

 slates, thi-Ugh it is probable that some at any rate of the footsteps 

 on the Triassic rocks are those of birds. So again the Amphibia 

 have been carried back from the Trias to the Coal-measures ; 

 Fish from the Old Red Sandstone to the Upper Silurian ; 

 Reptiles to the Trias ; Insects from the Cretaceous to the 

 Devonian ; Mollusca and Crustacea from the Silurian to the 

 Lower Cambrian. The rocks below the Cambrian, though of 

 immense thickness, have afforded no relics of animal life, if we 

 except the problematical Eo^oon Canadcnsc, so ably studied by 

 Dawson and Carpenter. But if pala:ontology as yet throws no 

 light on the original forms of life, we must remember that the 

 simplest and the lowest organisms are so soft and perishable that 

 they would leave " not a %vrai_k behind." 



Passing to the science of Geography, Mr. Clements Markham 

 has recently jjublished an excellent summary of what has been 

 accompli-hed during the half-century. 



But the progress in our kno» ledge of geography is, and has 

 been, by no means confined to the improvement of onr mips, 

 or to the discovery and description of new' regions of the eaith, 

 but has extended to the causes which have led to the present 

 configuration of the surface. To a great extent indeed this part 

 of the subject falls rather within the scope of geology, but I 

 may here refer, in illustration, to the distribution of lakes, the 

 phenomena of glacier-, the formation of volcanic mountain^, 

 and the structxire and distribution of coral islands. 



The origin and distribution of lakes is one of the most inter- 

 esting problems in physical ge igraphy. That they are not 

 scattered at random, a glance at the map is sufficient to show. 

 They abound in mountain districts, are comparatively rare in 

 equatorial regions, increasing in number as we go north, so that 

 in Scotland and the northern parts of America they are sown 

 broadcast. 



Perhaps a priori the first explanation of the origin of lakes 

 which would suggest itself, would be that they were formed in 

 hollows resulting from a disturbance of the strata, which had 

 thrown them into a ba-in-shaped form. Lake-basins, however, 

 of this character are, as a mailer cf fact, very rare ; as a general 

 rule lakes have not the form of basin-shaped synclinal hollows, 

 but, on the contrary, the strike of the strata often runs right 

 across them. My eminent predecissir. Prof. Ramsay, divides 

 lakes into three classes: — (I) Those which are due to irregular 

 accumulations of drif', and which are generally quite shallow. 

 (2) Those which are formed by moraines, and (3) those which 

 occupy true lia^ins scooped by glacier-ice out of the solid rock. 

 To the latter class belong most of the great Swiss and Italian 

 lakes. Prof. Ramsay attributes their excavation to glaciers, 

 because it is of omrse obvious that rivers cannot make basin- 

 shaped hollows surrounded by r' ck on all sides. Now the Lake 

 of Geneva, 1230 feet above the se.a, is 984 feet deep, the 

 Lake of Brienz is 1850 feet above the sea, and 2000 feet deep, 



so that its bottom is really below the sea-level. The Italian 

 lakes are even more remarkable. The Lake of Como, 700 feet 

 above the sen, is 1929 feet deep. Lago Maggiore, 6S5 feet 

 above the sea, is no less than 2625 feet deep. It will be 

 observed that these lakes, like many others in mountain regions-, 

 those of Scandinavia, for instance, lie in the direct channels of 

 the great old glaciers. If the mind is at first staggered at the 

 magnitude of the scale, we must remember that the ice which 

 scooped out the valley in which the Lake of Geneva now 

 repose-;, was once at least 2700 feet thick ; while the mjraines 

 were also of gigantic magnitude, that of Ivrea, for instance, 

 being m less than 1500 feet in height. Prof. Ramsay's theory 

 seems, therefore, to account beautifully for a large nuu.ber of 

 interesting facts. 



Passing from lakes to mountains, two rival theories with re''er- 

 ence to the structure and origin of volcanoes long struggled for 

 supremacy. 



The more general view was that the sheets of lava and scoria; 

 which form v> Icanic co-ies — such, for instance, as .5£tna or 

 Vesuvius — were originally nearly hori-^ontal, and that subse- 

 queni ly a force operating from below, and exerting a pressure 

 both upwards and outwards from a central axis towards all points 

 of the compass, uplifted the whole stratified mi s and made it 

 a sunie a ci nical form, giving rise at the same time, in many 

 ca-es, t'l a w ide and deep circular opening at the top of the 

 cone, called by the advocates of this h)polbesis a " crater of 

 elevation." 



This theory, though, as it seems to us now, it had already 

 received its deaih-blow from the admirable memoirs of Scrope, 

 was yet that most generally adopted fifiy years ago, because it 

 was considered that compsct and crystalline lavas could not have 

 consolid.ited on a slo] e exceeding 1° or 2°. In 1S5S, however, 

 Sir C. I yell conclusively showed that in fact such lavas c uld 

 consolidate at a consideralile angle, even in some cases at more 

 than 30', and it is now generally admilted that though the 1 eds 

 of lava, &c , may have su-lained a slight angular elevation since 

 their dep isition, still in the main, volcanic cones have acquired 

 their form by the accumulation of lava and ashes ejected from 

 one or more cra'ers. 



The prol'leuis jiresented by glaciers are of very great interest. 

 In 1843 Agassiz and Forbes proved that the centre of a glacier, 

 like that of a river, moves more rapidly than its sides. But how 

 and why do glaciers move at all? Kendu, afterwards Bishop of 

 Anncfv, in 1841 endeavoured to explain the facts by snpp >sing 

 that glacier ice enjoys a kind of ductility. The " viscous ther.rv' " 

 of glaciers was al-o adopted, and most ably advocated, by 

 Forbes, who compared the condition of a glacier to that of the 

 contents of a tar-barrcl poured into a sloping chaimel. We 

 have all, however, seen long narrow fissures, a mere fraction of 

 an inch in width, stretching far acr. ss glaciers— a condition 

 incompat.ble with the ordinary idea of viscosity. The pheiio- 

 memii of regelation was afterv. ards applied to the explanation 

 of glacier moti n. An obsenaiion of Faraday's supplied the 

 clue. He noticed in 1850 that when two pieces of thawing ice 

 are placed together they unite by freezing at the place of contact. 

 Foil nving up this suggestion, Tyndall found that if he com- 

 pres ed a block of ice in a mould it could be made to ossunie 

 any shape he pleased. A straight prism, for instance, placed in 

 a groove and submitted to hydraulic pressure, was bent into a 

 trans|>arent semicircle of ice. These experiments seem to have 

 proved that a glacial valley is a m-uld through which the ice is 

 forced, and to which it will accommodate itself, while as Tyndall 

 and Huxley also pointed out, the "veined structure of ice" is 

 produced by pressure, in the same marnerasthe cleavage of 

 slate rocks. 



It was in the year 1842 that Darwin pullished his great v ork 

 on " Coral Islands." The frinjing reefs of coral presented no 

 sprcinl difhuulty. They Cf uld be obviously accounted for by an 

 elevation of the land, so that the coral which had originally 

 grown under water, had been raised above the sea-'evel. The 

 circular or oval shape of so many re fs, however, each h .ving a 

 lagoon in the centre, closely surrounded by a deep ocean, and 

 rising but a few ftet above the sea-level, had long been a 1 uzzle 

 to th'e physical geogiapher. The favourite theor>- was that these 

 were the summits of submarine volcanoes on w hich the coral had 

 grown. But as the reef-making coral does not live at greater 

 depths than about twenty-five fathoms, the immense nnmber of 

 these reefs formed an almost insuperable objection to this theory. 

 The Laccadives and Maldives, for instance— meaning literally the 

 " lac of islands ' and the "thousand islands "—are a series of such 



