Dee. 8, 1870] 
NATURE 
107 

I could ; but long before I could get to where I had seen the 
ice, the whole lake as far as my view extended was open water. 
This almost instantaneous disappearance of a body of ice more 
than a foot in thickness can only take place in perfectly still 
weather. If there is any wind it breaks up, and the fragments 
are driven up against the ice which still holds together, and 
into the shores, where the rapidity with which it melts is not so 
striking. I never was fortunate enough to be actually in at the 
death. 
““There are also some curious facts connected with the air-holes 
which form themselves during winter. There are often particular 
spots where partial openings in the ice will be formed every 
winter. These I conceive to arise from warm springs, and to 
have no connection with air-holes properly so called, which are 
not confined to any particular locality, but may appear anywhere. 
There is always a good deal of air under ice, and you may often 
see it scattered about in small bubbles when the iceisthin. Itis 
probably air excluded in the process of crystallisation, and when 
there is added to it sundry gases formed from decaying matter 
in the water, it amounts during the winter to a considerable 
quantity. Such collections of air, like the bubble in a spirit- 
level, are in a very uneasy condition, and are rapidly transferred 
from one place to another on any casual disturbance of the level, 
giving rise to one of the numerous noises which are always more 
or less heard ona lake covered with ice—at least, we used always 
to attribute to this cause a peculiar groaning sound which was 
very common. Now, if there should be any casual inequality in 
the lower surface of the ice, the air will naturally collect there, 
and if it is aboye 32° F, which in so far as it consists of evolved 
gas it probably will be, the receptacle will be increased by thaw- 
ing. A dome-shaped cavity will thus be gradually formed, which 
will finally reach the surface ; air will escape from below, and 
the surface-water, of which there is almost always more oy less 
after the snow has fallen, will run down from above, wearing the 
little jagged channels which are characteristic of air-holes. The 
whole thing will then after a while freeze up again, leaving an 
indication of where the air-hole has been in the different colour 
of the freshly-formed ice. I have tried several such air-holes 
with an axe when first formed, and have always found them to 
lead to such a dome-shaped cavity. I remember on one occasion 
an otter frequenting a large air-hole which remained open for 
some time, and which must have been from a mile and a half to 
two miles distant from the nearest open water. How did he 
reach it? for no otter can travel that distance under water without 
access to air. The Indians say that they will go to greater dis- 
tances still under the ice, and that they always find air there. It 
is likely enough that there may be many such dome-shaped cavi- 
ties, which have not yet reached, and may never reach, the surface 
as air-holes, but one would imagine the air they contain to be 
not of the most wholesome character. However, this otter did 
frequent that air-hole for about a week, which it certainly did 
not reach by travelling on the ice, and thoughit had few chances 
of breathing there, in the daytime at any rate, it contrived 
during that period to elude the snares of a white man and an 
Indian, who wasted a good deal of time in looking after it. 
So far, the process of the formation of air-holes, if I am right 
in my explanation, is intelligible enough ; but sometimes they are 
formed in a manner which is difficult to account for. Upon one 
occasion I had crossed the lake to a friend’s house, about four 
miles off, and we had determined to start together next morning 
to our nearest town, but I had to go home first. I first went 
over by daylight, when there certainly was nothing unusual in 
the appearance of the ice, which might be four or five inches 
thick at the time, with a slight sprinkling of wettish snow on it. 
I returned home about eleven at night, and, as it was bright 
starlight, with only a few floating clouds, I should have noticed 
any change ; but I came straight across, and saw nothing to 
attract attention. But when I crossed again at daylight in 
the morning, in one part of the lake the whole surface was co- 
vered with air-holes—there must have been hundreds of them. 
At first I gave them rather a wide berth, but, on approaching 
one to examine it, I found it frozen up again, the clear ice in the 
hole, with very slight indications of the characteristic jagged 
edges, being the only sign that there had been an open air-hole 
there during the night. I had no axe with me to try whether 
they were connected with any cavity, but the appearance was as 
if holes of from two to five or six inches in diameter had been 
punched through theice. Ofcourse, we attributed it to electricity, 
as people will do anything which they do not otherwise understand, 
and I have never been able to give any more intelligible explanation 

of the phenomenon. There certainly had been some faint sheet 
lightning that night, a very unusual thing in winter ; but what 
connection, if any, there may have been between the two things, 
I cannot tell. 
“Ottawa, Sept. 15” “Joun LANGTON 
The Difficulties of Natural Selection 
J FIND, on looking again at Mr. Bennett’s article, that I have 
misrepresented him on one point, for which I beg to apologise. 
On his supposition, that the first twenty possible steps on the 
road to mimicry are absolutely useless, his argument will have 
some weight. This supposition, however, is entirely unsup- 
ported by facts. Very large variations of colour are exceedingly 
common in butterflies ; and when such variations are in the right 
direction, they must in some cases be useful. I believe myself 
that far less than fifty, or even twenty, steps of variation would in 
some cases produce very good mimicry. 
ALFRED R. WALLACE 
Cave-paintings by Bushmen 
My friend, Mr. George W. Stow, of Queenstown, South 
Africa, refers in a letter to the interesting subject of the old 
cave-paintings by the Bushmen, as follows: ‘‘ During the last 
thee years Ihave been making pilgrimages to the various old 
Bushman caves among the mountains in this part of the colony 
and Kaffraria ; and, as their paintings are becoming obliterated 
very fast, it struck me that it would be well to make copies of 
them before these interesting relics of an almost extinct race are 
entirely destroyed. This gave rise to an idea in my mind of 
collecting materials enough to compile a history of the manners 
and customs of the Bushmen, as depicted by themselves. I have, 
fortunately, been able to procure many fac-simile copies of 
hunting scenes, dances, fightings, &c., showing the modes of 
warfare, the chase, weapons, disguises, &c. ‘This promises to 
be a collection of very great interest. In some places it is 
astonishing to what a degree of perfection some of the wild 
artists had arrived. I have found three different series of paint- 
ings, one over the other ; and, as the most recent must be upwards 
of fifty years old, the undermost are most probably very ancient. 
The colours are very permanent, and would last for ages if not 
wantonly obliterated. Unfortunately, the Kaffir herds and 
others are constantly destroying them, and, by the time another 
generation has passed, few remains of them will be left.” 
The pigments used in the caves were derived from ochreous 
concretions abounding in some of the sandstones of the Karoo 
series of the interior of South Africa, as in the Rhenosterberg, 
Stormberg, and elsewhere. These concretions, when broken 
open, supplied the natives with paint-pots, and from among the 
several colours of yellows, browns, reds, &c., the chocolate was 
selected for painting the human form in the caves. 
T. RUPERT JONES 
5, Terrace, Yorktown, Surrey 
A Rare Fish 
A SPECIMEN of the Silvery Hair-tail ( Z7ichiurus lepturus ) was 
taken this morning at Seaton. It measures 2 feet 2 inches in 
length, and is in very good preservation, being only slightly in- 
jured on one side of the head. A specimen from the Collection 
of the late Mr. F. W. L. Ross, in th’s museum, is about the 
same size, and was taken on 6th August, 1852, off the Start 
Point, Devon. ‘The recorded instances in which this remarkable 
fish has occurred on the British coasts are very few, and the speci- 
mens obtained have generally been much injured. The present speci- 
men was brought to me to name by Mr. Frank Gosden, of the 
West of England Fish and Game Company, Queen Street, 
Exeter. W. S. M. D’URBAN, Curator 
Devon and Exeter Albert Memorial Museum, 
Queen Street, Exeter, December 3 
The Ceratodus Forsteri 
I aM inuch obliged to Dr. Sclater for his remarks on the new 
fish discovered by meas Ceratodus forsteri, and I take this 
opportunity to inform your readers who may feel interested in 
this matter, that I spoke of the animal as an amphibian, princi- 
pally because it is in the habit of leaving the water during the 
night. The works to which Dr. Sclater refers me are not at my 
command, and I adopted the generic term of Cera/odus because 

