ee EK 
= 
and are able to extract the metals thinly diffused through 
the soil, yet these products must become far dearer and less 
available for general use than now. Railroads and steam- 
ships, and everything that depends upon the possession 
of large quantities of cheap metals, will then be impos- 
sible, and sedentary agricultural populations in warm and 
fertile regions will be the best off. Population will have 
lingered longest around the greatest masses of coal and 
iron, but will finally become most densely aggregated 
within the tropics. But another and more serious change 
is going on, which will result in the gradual diminution 
and deterioration of the terrestrial surface. Assuming the 
undoubted fact that all our existing land is wearing away 
and being carried into the sea, but bya strange oversight, 
leaving out altogether the counteracting internal forces, 
which for countless ages past seem always to have raised 
ample tracts above the sea as fast as subaérial denudation 
has lowered them, it is argued that, even if all the land 
does not disappear and so man become finally extinct, 
yet the land will become less varied and will consist 
chiefly of a few flat and parched-up plains, and volcanic 
or coralline islands. Population will by this time neces- 
sarily have much diminished, but it is thought that an 
intelligent and persevering race may even then prosper. 
“They will enjoy the happiness which results from a 
peaceable existence, for, without metals or combustibles 
it will be difficult to form fleets to rule the seas or great 
armies to ravage the land ;” and the conclusion is that, 
“such are the probabilities according to the actual 
course of things.” Now, although we cannot admit 
this to be a probability on the grounds stated by 
M. de Candolle, it does seem a probability, at some 
more distant epoch, on other grounds. The great 
depths of the oceans extend over wide areas, whereas the 
great heights of the land are only narrow ridges and 
peaks ; hence it has been calculated that the mean height 
of the land is only 1,000 feet, while the mean depth of 
the sea is about 15,000 feet. But the sea is 2} times 
as extensive as the land, so that the bulk or mass 
of the land above the sea level will be only about one 
thirty-seventh of the mass of the ocean. Now, does not 
this small proportion of bulk of land to water render it 
highly probable that the forces of elevation and depression 
should sometimes cause the total or almost total sub- 
mersion of the land? Of such an epoch no geological 
record could be left because there could be no strata 
formed, except from the debris of coral islands, and such 
a period of destruction of the greater part of terrestrial 
life may have repeatedly occurred between the period 
when the several Primary or Secondary formations were 
deposited. At all events, with such a proportion of land 
and sea surface as now exists, with such a small bulk of 
land above the enormous bulk of water, and with no 
known cause why the dry land rather than the sea-bottom 
should be constantly elevate1, we must admit it to be 
almost certain that great fluctuations of the area of the 
land must occur, and that, while those fluctuations could 
not very considerably increase the area of the land 
they might immensely diminish it. There is here, 
therefore, a cause for the possible depopulation of the 
earth likely to occur much sooner than any cosmical 
catastrophe. 
The largest and most elaborate essay in the volume is 
that on the “ History of the Sciences and of Scientific 
Deseo) 
NATURE 
3 we a iee eee se 
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279 
Men for the last two Centuries.” In this the author 
endeavours to arrive at certain conclusions as to the pro- 
gress of science under different conditions and in different 
countries, the influence of political institutions and of 
heredity, and various other phenomena, by a method which 
is novel and ingenious. He takes account only of the 
men honoured as foreign associates or members by the 
three great European Scientific. bodies, the Royal So- 
ciety of London and the Paris and Berlin Academies, 
By this means he avoids all personal bias, and secures, 
on the whole, impartiality. The tables drawn out by this 
method are examined in every possible way, and the results 
worked out in the greatest detail. The main conclusion 
arrived at is the determination of a series of eighteen 
causes favourable to the progress of science; and it is 
shown that a large proportion of these are present 
in a considerable degree in countries where science 
flourishes, while they are almost wholly absent in bar- 
barous or sémi-civilised countries where science does 
not exist. 
Another interesting essay is that on the importance for 
science of a dominant language, and it contains some 
very curious facts as to the way in which the English 
language is spreading on the Continent. M. de Candolle 
believes that in less than two centuries English will be the 
dominant language, and will be almost exclusively used in 
scientific works, 
There are also short but very interesting essays on 
methods of teaching drawing and developing the ob- 
serving powers of children, on statistics and free will, 
and on a few other subjects of less importance, all of 
which are treated in a thoughtful manner, and illustrate 
one of the views on which much stress is laid in this work, 
viz., that the mental faculties which render a man great 
in any science are not special, but would enable him to 
attain equal eminence in many other branches of science 
or in any profession al or political career. 
ALFRED R. WALLACE 
HACKEL ON SPONGES 
Die Kalkschwimme. Eine Monographie. Von Ernst 
Hackel. 2 vols., with an additional vol. of 60 litho- 
graphic plates. (Berlin, 1872.) 
oh splendid contribution to the knowledge of the 
sub-class of Calcareous Sponges is worthy of the 
high reputation of Prof. Hackel. In the preface he 
speaks of it as one of the many results of the stimulus 
given to zoology by the Darwinian Theory ; and the list 
of those who have contributed the materials on which this 
monograph is based is honourable alike to the author and 
to the friendly helpers from our own and every other 
civilised country. It includes the names of Agassiz from 
the United States, Allman from Edinburgh, Percival 
Wright from Dublin, Barboza du Bocage from Lisbon, 
Lacaze Duthiers from Paris, the lamented Claparéde from 
Geneva, Eschmark and Sars from Christiania, Steenstrup 
from Copenhagen, and Lieberkiihn, Peters, Oscar 
Schmidt, Semper, von Siebold and many others from 
Germany. In addijion to this, the author has himself 
collected sponges in Heligoland, Nice, Naples, Messina, 
the Canaries and Mogador, Algesiras, Bergen, and the 
neighbouring Norwegian coast, and lastly in the Adriatic 
Sea. 
