2IO 



NA TURE 



[Dl CEMBER 30, 1897 



of hard carboniferous grit of a dark slate colour, except at 

 the western end, where this rock is interspersed with red 

 sandstone and shale and a few pockets of glacial drift. The 

 beach between the foot of the cliffs and low water consists of 

 rocks cut and furrowed by the action of the sea in perpetually 

 rolling about the large boulders which lie along its surface. 



Beyond Westward Ho the estuary of the Taw and the 

 Torridge commences, consisting of a vast expanse of sand 

 bounded by sand dunes. 



Large fragments of rocks have in the course of ages been 

 dislodged from the cliffs, the remains of which perpetually 

 rolled about by the waves of the sea during high tides, which 

 here rise to a height of 27 feet, have acted as instruments for 

 grinding their fellows, and battering the cliffs, and so produc- 

 ing the rounded boulders which now strew the beach through- 

 out its whole length for several miles, and a portion of which, 

 drifted along the shore of the bay, have become finally 

 heaped up in the Northam pebble ridge. 



In some parts of the cliff indents of considerable size have 

 been cut out, and across these the boulders have col- 

 lected, and been thrown up into ridges and banks. At 

 Abbotsham, about twelve miles from Hartland, there is such 

 a bank, the top of which is 9 feet above high water of spring 

 tides. This ridge or bank is about 160 feet wide, the boulders 

 of which it is composed varying in size at the top from about 12 

 inches in length by 4 inches in diameter to pebbles 3 inches in 

 diameter, the largest boulders weighing about 12 lbs., those at the 

 foot reaching to a length of 2 feet and weighing about 70 lbs. 

 Notwithstanding the large size of the boulders of which the 

 bank is composed its sea face is shaped into a ridge and 

 hollow, similar to other pebble ridges, the position of which 

 varies according to the height of previous spring tides. The 

 pebbles left on the shelf or hollow at the spring- tide level 

 average a smaller size than those at the other part of the bank. 



The boulders scattered along the beach all lie above the 

 level of low water of neap tides. The general direction of 

 movement is eastwards, but the boulders follow the line of 

 the coast and the set of the flood tide. This direction varies 

 round the bay from eastward to south-east, east agairtand 

 then north-east, and finally south-east. The direction of the 

 wind which drives the heaviest sea into the bay is from the 

 north-west. 



The Northam pebble ridge coiymences at the termination 

 of the cliffs, and runs in a north north-easterly direction for 

 upwards of two miles across a low flat plain which is below 

 the level of high tides, until it falls into some hummocks of 

 blown sand. It thus forms a natural embankment enclosing 

 a tract of 900 acres of sandy and alluvial grass land which 

 is used for grazing purposes, and also as golf links. After 

 running along the foot of the sand hills for a short distance 

 the pebble bank turns sharply to the south-east up the course 

 of the outfall of the two rivers, the boulders diminishing in 

 .size to pebbles and coarse sand. There is an outlying bed 

 of boulders, known as the Pulley, situated some distance from 

 the bank, on the edge of the low-water channel of the river, 

 but these appear to be a fixed deposit which neither in- 

 creases nor diminishes in size. 



The ridge is approximately 180 feet wide at the base and 20 

 feet high, the top being about 25 to 30 feet wide and 6 feet above 

 high water of spring tides. The boulders on the top of the bank 

 vary in size from about 12 inches in length by 6 inches in 

 diameter' to pebbles an inch in diameter, the average size being 

 about 8 inches in length by 4 inches in diameter, the largest 

 being about 12 inches long and weighing from 40 to 50 lbs. 

 At the foot of the bank are to be found boulders measuring from 

 15 to 18 inches in length and weighing from 100 to 150 lbs. 

 The size of the boulders does not vary much throughout the 

 length of the bank. The greatest collection of small stones 

 • appears to be on the shelf or hollow at the level of spring tides, 

 where the pebbles vary from about half an inch to four inches in 

 diameter. Some of the larger boulders have been drifted quite 

 to the far end of the bank. 



The boulders consist entirely of the same description of slate- 

 coloured carboniferous grit as the cliffs from Hartland to Abbots- 

 ham are composed of, and there can be no doubt that they have 

 drifted from this part of the coast. At the commencement of 

 the ridge there are fairly numerous samples of shale and red 

 sandstone pebbles from the cliffs between Westward Ho and 

 Abbotsham, but these gradually disappear further along the 

 ridge, the softer rock of which they are composed evidently not 



being able to withstand the constant grinding process produced 

 by the wave action of the tides and wind. From the foot of 

 the bank to low water the beach is covered with sand, which 

 dries from a third of a mile at the south end to three-quarters 

 of a mile at the northern end. 



There is a very slow but continuous drift or movement of the 

 boulders along the bank northwards. The progress of the 

 ridge being stopped by the sand hills, the bank has bifurcated 

 at this point, a new or double bank now forming, a circumstance 

 which has occurred within the knowledge of those who have 

 known the bank all their lives. 



The boulders composing the ridge are in perpetual motion 

 during the time that the bank is covered by the sea at spring 

 tides. Even in calm weather in summer the whole face of the 

 bank is continually changing under the influence of the wave 

 action of the flood and ebb tide, which is of sufficient force to 

 cause the movement of even large boulders. Observers who 

 have carefully watched this movement and marked individual 

 stones find that they are never in the same place two tides run- 

 ning, and each spring tide leaves its impress in a hollow and 

 ridge at high-water mark. 



In hea%'y on-shore gales these ridges and hollows are ob- 

 literated, and the face of the bank is pulled down seaward, 

 the extent to which this is carried depending on the force and 

 duration of the gale. After the storm, and when the height 

 and force of the waves have subsided, the pebbles begin to 

 move back again ; the contour of the bank becomes more steep, 

 and is soon restored to its normal condition. 



During the winter at the end of 1896 there was a succession 

 of westerly gales, culminating in a very heavy gale from the 

 north-west. The bank was torn down and so lowered that 

 the waves broke over it and inundated the enclosed land. Some 

 of the largest boulders were thrown over the top of the ridge 

 and hurled a considerable distance inland, where they now 

 remain as a witness to the force of the gale. The disturbance 

 of the boulders was so great under the action of the waves, that 

 after the gale it was found that the base of the bank was moved 

 ten yards inland, the clay bed on which it had rested previously 

 being exposed. A somewhat similar movement took place 

 during a gale about twenty years previously. 



The peculiarity of this pebble ridge, and the way in which it 

 differs from ordinary shingle banks, is in the large size of the 

 boulders drifted along the coast, and heaped up by the action 

 of the waves and tides. W. H. Wheeler. 



RANDOM SELECTION. 



'X'HIS memoir^ is the first of a series dealing with the problem 

 •^ of selection, namely the measurement of the changes in the 

 characters of a race, when selection has acted upon anyone, two, 

 or more of them. The problem mathematically differs consider- 

 ably according to the nature of the selection. But in all cases 

 the general result is the same, the selection of any organ, 

 whether by size, variability, or correlation with other organs, 

 changes the sizes, variabilities, correlations of all other organs, 

 whether directly correlated with the first organ, or only indirectly 

 correlated with it owing to correlation with other organs which 

 are correlated with the first organ. (A and C may have no cor- 

 relation with each other, but both be correlated with B, e.g. two 

 parents in the absence of sexual selection and their offspring. ) 

 The chief types of selection which have to be treated in- 

 dependently are : — 



(i. ) Randovi Selection or Sample Selection. — The isolation of a 

 group out of a larger population. This will generally have 

 characters divergent from those of the general population, but 

 which form in themselves a correlated system of divergences. 



(ii.) Epidemic Selection.— '$>e{e:c\\on which takes place so 

 quickly that the growth or reproduction of the population may 

 be neglected. For example, a severe winter or a pestilence. 



(iii.) Auxetic Selection, or long-continued selection which 

 allows during its action for growth, but not for reproduction. 

 For example, diseases of childhood. 



(iv. ) Gonimic Selection, or long-continued selection which allows 

 during its action for reproduction. For example, physical and 

 mental qualities, pressure of other populations. These forms of 



1 Contributions to the Mathematical Theory of Evolution. IV. On the 

 Probable Errors of Frequency Constants, and on the Influence of Random 

 Selection on Variation and Correlation, by Karl Pearson and L. N. G. 

 Filon. (Royal Society, Nov. 25, 1897.) 



NO. 1470. VOL. 57] 



