May 14, 1 8 74 J 



NATURE 



25 



mere trace of it. And the flowers being no longer cut off by 

 the birds supports my belief tliat the nectar is one chief attrac- 

 tion to thera ; the 0%'ules without the sauce not being worth 

 the gathering. I may add that as the primrose is a dimorphic 

 plant, these non-nectariferous flowers would be sterile, for they 

 would not be visited by insects. — CD. 



Mr. Spencer and ^ priori Axioms 

 I QUITE agree with Mr. Spencer that argument between us 

 will not be to much purpose ; but il should be noted that my 

 principal " exemplificntion of unconsciously-formed preconcep- 

 tions " was of Mr. Spencer's own choosing, namely, Newton's 

 "Second Law of Motion,'' which, if I understand him aright, 

 may now be described as "a consciously-formed hypothesis con- 

 cerning the relation between weight (force?) and motion." Only 

 demurring to the word "hypothesis," and leaving it to Mr. 

 Spencer to reconcile this with his former declaration that the 

 law in question is an "immediate corollary" of one of these un- 

 consciously-formed preconceptions, it seems to me there is little 

 left to argue about. Robt. B. Hayward 



Harrow, May S 



Mr. Spencer does not state his arithmetical illustrat'on very 

 exactly. He implies that there is a certain truth which the 

 savage is incapable of understanding concerning which the 

 schoolboy makes a mistake, but that there is present in the 

 civilised adult a consciousness of its logical necessity. It does 

 not appear distinctly what that truth is. 



The most obvious interpretation of what is printed is, that 

 Mr. Spencer refers to the local value of figures in the Arabic 

 system of notation : this is probably not what is meant. 



Two other interpretations suggest themselves. The sum of 

 seven and five is the same number whatever be the things to 

 which the feven and five refer; or else the more particular state- 

 irent that the sum of seven and five is the same as the sum of 

 ten and two. It is not apparent that either of these is intended. 



To say that seven and five make twelve without implying 

 something about twelve other than the statement that it is seven 

 and five, seems a proposition so purely verbal that it is difficult 

 to see how the recognition or non-recognition of it illustrates 

 the grounds of belief in physical laws. 



Not a Metaphysician 



The Glacial Period 



In the many kind and favourable reviews of my book, "The 

 Naturalist in Nicaragua," exception has been generally taken to 

 my speculations on the extent and effects of the ice of the glacial 

 period. The subject is a large on^, and too litile of my time can 

 be given to scientific inquiry to allow me to hope that I can deal 

 with it in detail for some years to come ; but as it appears that I I 

 have not expressed my views wi'.h sufficient clearness and have 1 

 been misunderstood by some of my critics, I shall be glad of an 

 opportunity to state them with distinctness and brevity. 



1. A' the present sea-level, the ice extended, in the northern 

 hemisphere, from the Pole to lat. 39° in America, to about the 

 valley of the Thames in England, to lat. 50° in central Europe, 

 and to lat. 52° in north-western Asia. Along the high lands of 

 America it reached to the tropics, and in Central America all 

 the land lying over 2,000 ft. above the sea supported ghciers. I 

 do not contend that the present low Lands of tropical America 

 were ever covered with ice, and it is on the mountain chains of 

 that continent alone that I believe it nearly reached to the equator. 



2. The ice was thickest over the American continent, not be- 

 cause it was coldest there, but because the great evaporating area 

 of the Pacific lay to the south-west of it and the counter trade- 

 wind swept across it and precipitated the moisture with ^\'hich it 

 was laden. Siberia was equally cold, but the upper moisture- 

 beating currents of air were intercepted by the Himalayas, the 

 Kuen Lun, and the Altai Mountains. It was thickest in 

 America for the .same reason that it is thicker on the summits of 

 the Pyrenees than on similar heights on the Caucasus, and 

 thicker on the southern than on the northern slope of the Hima- 

 layas, not because of greater cold, but of greater precipitation. 



3. The immense accumulation of ice in the extreme north of 

 America and Europe must have overflown and filled the polar 

 basin even if it had not independently collected there ; but the 

 precipitated moisture would not have frozen on the continents if 

 the climate had not been much colder then than now ; and the sur- 

 face of the Arctic Ocean must have been frozen over, and as capable 

 of sustaining accumulations of snow as the solid land itself, 



even if that ocean was not displaced by the ice flowing into it 

 from the northern extremities of the continents. 



4. Probably the ice was not thickest at the Pole, but formed 

 a ridge of varying height at unequal distances from it ; for, as 

 we have seen, it would not be thickest where it was coldest, but 

 where there was most precipitation, and the south-west winds 

 would part with their moisture long before they reached the 

 Pole. 



5. Whilst we can follow on the land the marks left by the ice of 

 the glacial period, and map out its former boundaries, we can 

 only speculate on its extent over the areas now covered by the 

 sea. We have, however, some evidence. The Hebrides 

 and the extreme north-east of Scotland were ovei flown by ice 

 that came from the north-west, and the bed of the North Atlantic 

 must have been filled so far at least, or to about lat. 59° ; and 

 taking in*o account the much greater quantity of ice lying on 

 America than on Europe, it is not an extreme supposition that 

 on the western side of the Atlantic the bed of the ocean was 

 occupied by ice to lat. 45°. 



6. One of the principal effects of this great advance and ac- 

 cumulation of ice, not yet taken into consideration by geologists, 

 was an interruption to the drainage of all countries whose rivers 

 flowed northwards. The great plain of Siberia was, I believe, 

 occupied by an immense lake caused by the blocking up of the 

 whole of the watershed to the north. In western Europe this 

 interference with the drainage of the land took place, even if we 

 do not accept the theory of an ice-cap, but held with some 

 geologists that the ice descended only from existing chains of 

 mountains. All the rivers of northern Germany must have been 

 dammed back by the ice descending from the Scandinavian 

 mountains. One of the most important changes was effected in 

 the German Ocean. Its northern half was filled with ice, 

 from the mountains of Norway and Sweden, from Scotland and 

 northern England. As we know that at this time the Straits of 

 Dover did not exist, it is evident that the southern poition of the 

 bed of the German Ocean must have been filled by a great fresh- 

 water lake, varying in extent during the advance and retreat of 

 the ice, into which flowed all the water of the melting ice, and 

 all the rivers that now run into the same area. 



7. There is no satisfactory evidence of the intercalation of a 

 warm pericd between two glacial ones, though doubtless there 

 was more than one retreat of the ice, during which a temperate 

 climate prevailed in regions glaciated before and afterwards. 

 The intermingling of the remains of northern and southern 

 mammalia in the gravels of south-eastern England arose, pro- 

 bably, as explained by .Sir Charles Eyell, by a northern and a 

 southern fauna having migrated to the district at different seasons 

 of the year. 



When the German Ocean was blocked up lo the north by ice, 

 a great river must have run to the south through what are now the 

 Straitsof Dover and the English Channel, receiving into one stream 

 the waters of the Rhine, the Thames, the Humber, and the Somme. 

 How far that river ran southward would depend upon the rela- 

 tive heights of the land and the sea. It must have run into a 

 comparatively warm ocean, for the effects of the warm currents 

 of water coming from the tropics, instead of .as at the present 

 time entering the polar basin, would be confined to and intensi- 

 fied in more southevn latitudes, and they would thtn, as now, be 

 deflected upon the western coa.st of Europe. Up this river the 

 hippopotamus and the southern species of rhinoceros and ele- 

 phant may have come in summer and autumn, whilst the mam- 

 moth, the woolly rhinoceros, and the musk ox came from the 

 north in winter. 



8. The theory of the damming-up of many rivers throws 

 much light on the difficult question of the formation of the high 

 and low level gravels and the loess. The lake occupying the 

 area of the German Ocean must have stood much higher in 

 spring and early summer than it did later on in the year and 

 in winter ; and the levels of the lower parts of the rivers running 

 into it must have been aftected by its rise and fall. If we can 

 suppose that the hippopotamus only came up the river when it 

 was low in the latter part of summer, or in the autumn, we can 

 understand how its remains are only found in the low-level 

 gravels of the Thames and the Somme ; though it is also possi- 

 ble that they may belong to a later and milder period when the 

 ice had retired so far back that the great lake partly drained to 

 the north around Scotland. 



9. The glacial period probably existed in both hemispheres at 

 the same time. First, because we can trace the evidence of the 

 existence of ice along the high lands of America into the north- 

 ern tropics until it nearly inosculates with that coming down 



