June 28, 1894] 



NATURE 



199 



sion. Only an insignificant addition to the estimate of 7000 

 years can therefore be required by the diversion of the waters of 

 tbe upper lakes. 



The view held by Taylor, Spencer, and L.awson, that the 

 high shore lines around the great Laurentian lakes are of marine 

 formation, seems to be inconsistent with the total absence of 

 marine fossiliferous beds overlying the glacial drift in all that 

 region. So far as the sea did extend, afier the further recession 

 of the ice-sheet permitted it to come into the St. Lawrence 

 and Ottawa valleys and into the basin of lake Champlain, 

 marine fossils abound ; but none are found above the Thousand 

 Islands at the moath of lake Ontario. We may therefore con- 

 fidently accept the Niagara gorge as a measure of all the time 

 since the departure of the icesheet from the northern United 

 States. 



In a recent paper in the Journal of Geology (vol. ii. p. 142, 

 February-March, 1894) Mr. Andrew M. Hansen, of Norway, 

 notes the approximate concurrence of about thirty independent 

 measurements and estimates of the duration of the Post-glacial 

 period, which have been made in North America and in Europe, 

 all coming within the limits of 5000 and 12,000 years. He 

 accordingly says : " With full regard to a legitimate calculation 

 of probabilities, it may be predicated that the number of 7000 

 to 10,000 ye.Trs is as nearly an exact estimate of the duration of 

 Post-glacial time as can ever be expected." 



Minneapolis, Minn., June 9. Warren Upham. 



The Teeth and Civilisation. 



None of the writers of the interesting letters which have ap- 

 peared upon this subject seem to have kept before them a 

 distinction which is of the utmost importance in its investigation, 

 and which I should like to state, in order that the attention of 

 those who, like Mr. Wenyon, have opportunities of observation 

 of any segregated comtnunity, may be drawn to it. 



Dental caries is very prevalent, and its increase seems to be 

 very rapid, so that the last few generations show a marked in- 

 crease ; at least so it is generally believed. 



liut its victims may be divided into two groups, namely, those 

 whose teeth are apparently perfect in their construction, but 

 nevertheless fall a prey to caries, and those whose teeth show, 

 iie trained eye, clear evidence of si ructural weakness. 

 ,1 the latter class present a problem in heredity, and for 

 ..41 luus reasons are likely to be more interesting 10 the readers 

 of Natl'RE than the former, I will dismiss these with a very 

 few words. 



There is good reason for supposing that the proximate 

 cause is to be found in vitiation of the oral secretions, as caries 

 often occurs in an extreme degree after diseases of the digestive 

 tract, and examples such as those quoted by Mr. Wenyon are 

 probably to be explained a- due to dyspepsia induced by the 

 unhealthy way o( feeding. 



To the explanation that the enamel may be cracked by 

 alternations of temperature which could be borne in the mouth, 

 I am not inclined to attach importance. In the first place, 

 as a matter of experiment, I have failed to crack enamel by 

 plunging teeth alternately into boiling and ice-cold water, and, 

 as a matter of clinical experience, teeth do not decay along the 

 cracks which from some cause are common in the enamel, but 

 in natural pits of larger size, or on surfaces of contact ; the 

 i cause is to be sought either in decreased power of resistance, or 

 in the intensification of deleterious influences. 



Abnormal conditions of life are known to deleteriously affect 

 the teeth of animals ; for example, stall-fed beasts are more 

 liable to diseased conditions in the mouth than those which are 

 fed up in rich pastures ; and it has been pointed out by Mr. 

 I Bland Sutton, that some animals in the Zoological Gardens suffer 

 I in this way, notably the Lemurs, whose teeth frequently loosen 

 and fall out. 



It is rare to meet with good teeth in children whose parents 

 have had had teeth, and peculiarities of form in the teeth and 

 jaws are often inherited with curious exactitude. liut it is quite 

 common to meet with instances of healthy parents with good 

 teeth bearing a family of children, also apparently healthy 

 and well-grown, whose teeth, although to the casual observer 

 normal in shape, sizr, and general aspect, are to the eye of the 

 dentist doomed to early destruction, and speedily undergo it. 



These teeth have an appearance somewhatdifficult todescribe ; 

 they have a glassy look, are more translucent than they should 

 be, are softer, and are believed, though the proof is not com- 



NO. 1287, VOL. 50] 



plete, to be somewhat deficient in their proper proportion of 

 lime salts. This kind of tooth is \ery apt to run through a 

 whole family, and its causes must be sought either in some con- 

 dition of inheritance, or if it be due to anything acting upon the 

 individual, it must be something which commences to act im- 

 mediately after birth. Comparative anatomy teaches us that 

 the teeth are less variable than the jaws. In long-muzzled dogs 

 the teeth are spaced ; in short-muzzled dogs they are crowded, 

 reduction in size having gone on faster in the bones than in the 

 teeth ; and the same thing is observed in the human subject. 



Moreover in animals, whilst some variability is often observ- 

 able in the teeth, that variability does not lake the form of 

 structural difference, but only of differences of .size and shape. 



Again, in rickets, where the bones are starved of lime salts, 

 the teeth contrive to get more than their share ; it is therefore 

 not a little remarkable that we should find the teeth, and 

 apparently the teeth alone, to have deteriorated in one 

 generation. 



On the other hand, it is equally difficult to find any cause 

 which shall have operated alike upon all the children of a family 

 if we reject inheritance as being at the bottom of it ; it does not 

 appear to be determined by the greater frequency of hand- 

 feeding, as I know of instances in the same family where some 

 children were nursed and others were not, and yet their teeth 

 were alike of the poor structure to which I have alluded. 



Upon the whole, I am rather inclined to attribute it to some 

 causes operating shortly after birih, rather than before it, for 

 the milk teeth, which are well started inutero, are far less liable 

 to structural variation than are the permanent teeth, whose 

 calcification is mainly effected after birth ; but I need hardly say 

 that the period of occurrence does not by any means exclude 

 inheritance. 



However the question, interesting and important as it is, is 

 not so simple as some of your correspondents imagine, and 

 there is a considerable amount of literature upon the subject 

 which seems to have escaped them, some of it accurate and 

 valuable, some of it quite the reverie. 



Charles S. Tomes. 



Electrical Theory of Vision. 



In my letter to Prof. Lodge, published in your last issue, I 

 notice a printer's error, which I think should be corrected, as 

 it gives an entirely wrong meaning to the sentence in which it 

 occurs. As it stands it reads as follows: "The energy thus 

 lost by the tissues was then mpprcssed from without by the 

 vibrating fingers," whereas I said the energy was supplied 

 from without by the fingers, the idea being that the shaking 

 back of the eyes to their normal state of rest, evinced by the 

 sensation of darkness, is perfectly analogous to the tapping 

 back of Prof. Lodge's "Coherer" to its normal position, 

 evinced by the return of the galvanometer needle to zero. 



E. Obach. 



Old Charlton, Kent, June 23. 



CLIMBING AND EXPLORATION IN THE 

 KARAKORAM-HLMALA YAS.^ 



THE mountain district explored by Mr. Conway lies on 

 the southern side of the watershed of the Karakoram 

 chain, and is drained by tributaries of the Upper Indus. 

 For most of the time he was in lialtistan, but ended his 

 journey by a visit to Leh. Here, at about 11,500 feet 

 above sea-level, is a small meteorological observatory, 

 which enabled Mr. Conway to check his observations by 

 a comparison of barometers. Besides himself, the party 

 consisted of the Hon. C. G. Bruce, Mr. McCormick, the 

 artist, Messrs. Eckenstein and Roudebush, Matthias 

 Zurbriggen, the well-known guide from Macugna, four 

 Gurkas, who took readily to ice work, and one or two 

 two other native attendants, with, of course, a consider- 

 able but variable patty of coolies. 



As the author states, the party spent in all eighty-four 

 days on snow or glacier. They were often encamped at 



' "OiinbinK and Exploration in the Kar.-ikoratn-Himalayas." Ky 

 Willfaii M.-irlin Conway, M.A., F.S..-\.. &c. With soj lllu^lratioQS by 

 A. P. McCormick, and a Map. (London ; T. Fisher t'nwin, 1394.) 



