Sept 5, 1889] 



NATURE 



453 



ject of the sensation of motion, has very clearly proved that this 

 is not the case. Everything we know as to this sense leads us 

 to look for its organ in the head. And there is an organ which, 

 to some extent, at all events, seems to be what we are in search 

 of. The macula aaisiica in the utricle is a spot well furnished 

 with nerves, and we have n )t found out any special function for 

 it. There is a similar Macula in the saccule, that other mem- 

 branous bag contained in the vestibule. Mach has suggested 

 that the macula of the utricle may be the organ by means of 

 which we perceive acceleration of translatory motion. Let us 

 look at it, and see how far it is fitted to act as a level. Its length 

 is stated by Prof. Schwalbe to be about -^ of an inch, and its 

 breadth a little less. According to the same observer, it covers 

 a part of the floor, the anterior wall, and a part of the external 

 wall of that part of the utricle called the "recess." Its nerves 

 end, as has already been stated, in hair-cells, and these are 

 covered by a gelatinous substance filled with a fine powder of 

 crystals of carbonate of lime. What is interesting to us in this 

 description is that it looks in three directions, and that the whole 

 of it is covered with a powder of considerably greater density 

 than the fluid (the endolymph) with which the utricle is filled. 



Let us try to imagine a model of this structure. Let us take 

 a box of glass, so that we may see what goes on in the inside of 

 it. Let us put on a part of the bottom of the box, on the end 

 of it, and on a part of the side of it, a layer of thin jelly mixed 

 with fine sand. Fill up the box with water, and put on a lid. 

 We shall find that we have an apparatus that does to some extent 

 answer the purpose of a level. When we change its inclination, 

 the jelly but for the sand would indeed have very little tendency 

 to change its position, but the sand, being specifically heavier 

 than the water will, and will either move through the jelly if 

 that is thin enough, or pull xhz jelly with it. In any case, a 

 change of position of the box will involve a change in the relative 

 position of its contents. In the actual case of the iitacjila acustica 

 such a change in the relative position of the sand and the hair- 

 cells must give rise to an irritation of the terminations of the 

 nerves, and send a message to the brain. We cannot as yet 

 work out all this in detail, as we can the way in which the 

 canals give us information as to the acceleration of rotational 

 motion, but we know enough to turn our attention to the subject ; 

 and we may hope that, by more accurate study of the sensation 

 phenomena, and by comparison of them with the anatomical 

 facts, this important and interesting physiological question may 

 be satisfactorily answered. 



ON THE GEOLOGICAL HISTORY OF THE 

 PREHISTORIC FLORA OF SWEDEN. 



"PIFTY years have passed since the Danish Professor, Japetus 

 ■*■ Steenstrup, presented to the world his masterly researches 

 on the history of Denmark's peat-bogs. These researches clearly 

 demonstrated that the forests of Denmark had suffered remark- 

 able variations. The oldest forests had consisted chiefly o( 

 aspen (Foptcltts tremula), next, for a long period, of Scotch fir 

 {Pinus sylveslris), then of oak, and, finally, of alder (Aliius 

 i^lutiuosa). Remains of the beech — now Denmark's chief species 

 of tree — are, however, entirely absent from the peat-bog. Con- 

 sequently, it must have been the last to immigrate. It was a 

 natural surmise — and one even advanced by Steenstrup— that the 

 changes referred to were connected with a gradual softening of 

 the climate, a view defended, too, by Prof. Forchhammer. 



But Steenstrup's researches were in advance of their time ; 

 glacial geology was only in its infancy, and voices were there- 

 fore raised both in Denmark and Sweden claiming to interpret 

 these changes in the forest vegetation as one of Nature's great 

 systems of evolution, whereby one variety, so to speak, prepared 

 the soil for the next comer, without any reference to climate. 

 However, this view has now but an historical interest. For, 

 since our knowledge of the geology of the Glacial age has become 

 more and more enlarged, and since remains have been found 

 in Scania of a true Arctic flora embedded in the fresh-water clay 

 deposits of that province, a return to Steenstrup's theory that 

 the changes of climate and forest vegetation were related was 

 but natural. Indeed, the same Arctic flora was shortly dis- 

 covered underneath the aspen layer in tlie Danish peat -bogs, 

 sj ihat the aspen flora cannot be regarded as the first alter 

 the Glacial age. By degrees, as the ice melted, the denuded 

 soil was invaded by the Arctic flora from the south. First, 

 when the climate became still milder, and a forest vegeta- 



tion could flourish, the aspen and the birch immigrated, and in 

 turn the pine, oak, and alder. Each one formed in its day the 

 forest of Denmark, and they were naturally accompanied each 

 by its own peculiar undergrowth of shrubs and plants. Enor- 

 mous ages elapsed between these events. The remains of the 

 peat-bogs show that generations upon generations of grand firs 

 flourished before the oak immigrated and before the pine flora 

 was at last ousted. And in its turn the oak reigned supreme 

 during countless ages, until that, too, was extinguished by the 

 alder and the beech. During the fir period, the men of the 

 Stone Age spread themselves over the land ; when the fir was 

 supplanted by the oak, the Bronze Age began. If we bear in 

 mind how exceedingly slow the extension of the oak and the 

 beech is, we can form some idea of the imniense time that must 

 have elapsed since the Ice age. 



Therefore the flora of Denmark consists entirely of the off"- 

 spring of immigrated plants. Many of the species which 

 appeared in the country did not remain ; to them Denmark 

 was only a station on the road towards higher latitudes. This 

 was, for instance, the case with the greater portion of the Arctic 

 flora, as well as with the fir. PVom the beginning of the his- 

 torical period the beech has flourished throughout the whole 

 country. 



In Sweden, in districts rich in calcareous matter, tuff strata 

 are deposited from springs which, by their contents of carbonic 

 acid, contain carbonate of lime. When calcaieous water is 

 exposed to the air, the carbonic acid gas evaporates, and the 

 carbonate of lime is precipitated in the lorm of a white deposit, 

 which soon hardens to stone. Such a deposit is therefore par- 

 ticularly natural around the mouth of a spring or in some pool 

 inio which the calcareous water is discharged. Consequently 

 the leaves or other remains of plants growing around are 

 covered, soon after falling into the water, with a thin coating of 

 chalk, and although they are by degrees destroyed, this imprint 

 in the chalk remains — often so distinctly that the finest fibres of 

 a leaf may be traced. 



A necessity for the formation of calcareous tuff is therefore the 

 presence of calcareous rocks, whence the water may draw the 

 lime. And, indeed, with us all calcaieous tuff deposits, as far as 

 we know, are confined to where the chalk formations are richest, 

 as, for instance, Scania, Western and Eastern Gothia, Jemtland, 

 Angermanland, and Asele Lappmark. 



The greatest in extent and richest in leaf imprints are the old 

 well-known tuff strata at Benestad, in Scania, north of the town 

 of Ystad. They were formerly largely worked for building 

 purposes, as, for instance, for several churches. The tuff is 

 referred to by early writers as rich in leaf imprints, but 

 in their writings a serious error crept in, viz. that even 

 leaves of the beech weie imprinted. This is wholly without 

 foundation. 



In consequence of the quantity of stone removed, it is now 

 impossible to fix precisely the nature of the stratification, and 

 what we know on this point is due to the particulars supplied by 

 Baron Claes Kurck, who carefully examined the strata. His 

 researches fully confirm the views of the writer, expressed as far 

 back as 1872, viz. that the oldest strata we/e deposited whilst the 

 aspen was the predominating tree in the districts, and before the 

 fir had immigrated. Kurck has also found here traces of birch, 

 grey sallow {Saltx cinerea), and possibly the common sallow 

 {Salix caprea). Above this stratum we come to the fir, deposited 

 when that predominated. The imprints of the fir, in the shape 

 of needles, branches, bark, and cones, show that this tree grew 

 clo.-e by the springs. Most of the tuff dates from the fir period, but 

 during ihe same age other species of trees gradually immigrated, 

 of which indications are found in the lower parts of the fir 

 deposit. From these the National Museum possess a rich col- 

 lection, chiefly made by Nordenskiold in 1873. It contains 

 several rare species of plants, and the imprints of leaves are to 

 remarkably clear that the collection is one of the greatest orna- 

 ments of the palasontological section. 



Of the trees which flourished contemporaneously with the fir, 

 we learn from Kurck that the birch, mountain ash (Sordus 

 aucuporia), Salix caprea, and Salix aurita, were the oldest, and 

 the hazel but little younger. To somewhat later strata he refers 

 dog- wood {Cornus sanguiriea) and berry aldtr i^Rhammts fran- 

 gula), and he believes that the remains found of Dutch rush 

 \Equise!um hyemalc), and the gueHer rose {Viburnum opulus) 

 also date from the same period. If this be the case, it is prob- 

 able that some leaves in the Nordenskiold collection of haw- 

 thorn (perhaps Crattrgus monogyna) also belong to this stratum. 



