APRIL 23, 1914] 
of the tectonics of the earlier Palzeozoic rocks describe 
folding and shearing which approach this average, 
though, of course, there are some extensive areas 
where the disturbance has been much less. When we 
come to the Archzean rocks the case is even stronger. 
A very simple calculation shows that the amount of 
contraction suggested above implies only a rate of 
shrinkage so slow that if going on at present it would 
fail to be detected by the most refined geodetic mea- 
surements repeated after a century. A contraction of 
the earth’s radius of only three inches a year, which 
would cause a 20 per cent. diminution of the radius in 
about twenty million years (and this is probably a low 
estimate for the age of the Cambrian strata) would 
only change the absolute length of geodetic lines by 
less than one part in a million in a hundred years. 
Observations of the lunar parallax, which afford 
another theoretical means of detecting changes of the 
earth’s radius, would have to be repeated at intervals 
of twenty-five thousand years in order to detect a 
single second of diminution, even if we could assume 
the moon’s mean distance to be constant throughout 
this period. 
If the hypothesis of a continuous contraction of the 
earth’s radius at an average rate of somewhere about 
three inches a year throughout geological time is 
entertained, it not only furnishes a plausible explana- 
tion of the prevalent folding of the ancient rocks, but 
also tempts one to indulge in a number of other 
speculations, such as, for instance, how far the flying- 
powers of the great Mesozoic reptiles may have been 
influenced by the lesser value of gravity at the earth’s 
surface with a larger terrestrial radius and a greater 
speed of rotation. But collateral issues may well rest 
until the main point has been argued, and as to this 
I should like the opinion of geologists whose experi- 
ence of the older formations has been greater than 
my own. I am at present surveying in a disturbed 
country where I should certainly have had to correct 
my base-line very considerably for dislevelment if I 
had measured it along the planes of stratification of 
the rocks, and this circumstance has suggested the 
question to my mind on reading Mr. Fisher’s letter. 
Joun Batt. 
Wadi Shellal, Sinai, March 31. 
Zoological Classification. 
THE complaint, thus entitled, of Mr. H. C. William- 
son in Nature for April 9, p. 135, is a fairly common 
one just now, and his way of putting it suggests some 
remarks frog the other side. 
The zoological classification of this or any other 
day is ‘‘satisfactory’’ in proportion, first, as the prin- 
ciples of that classification commend themselves to the 
general intelligence of zoologists; secondly, as the 
classification is in agreement with those principles. 
The multiplication of the units to be classified, 
whether subspecies, species, or genera, can have no 
bearing on the validity of the classification. 
The object of classification is the arrangement, but 
not the nomenclature, of the units. 
The older genera have been subdivided, not merely 
because the number of species has proved unwieldy 
(indeed, if the necessary bases be absent, such genera 
cannot be split, however much it be desired), nor 
merely because it has proved possible to group the 
species (for in this case subgeneric divisions are 
adequate), but because study has shown that the 
species formerly included in a genus have not really 
the close interrelationship which such _ inclusion 
implies, but are of very diverse descent. In nomen- 
clature as affected by classification most of the serious 
difficulties which happen to have come under my own 
NOL 2e20, VOL. 93) 
NATURE 
189 
notice are due to our increased, but still imperfect, 
knowledge of origins. 
The real trouble and source of disagreement between 
zoologists is that some (and especially the ‘‘ applied” 
workers) look at taxonomy with the eyes of the old 
school, which regarded only the amount of resem- 
blance in structure, while others follow the new school 
which seeks to express history and relationship. In 
a word, there are two sets of principles, and our 
classification is, if not halting between the two, at 
least passing rather lamely from one to the other. 
And as for nomenclature, the trouble is that the 
Linnean system was devised for a classification on the 
old principles, and one often doubts how far it is 
applicable to the new method. 
When the nature of our difficulties is clearly under- 
stood, nobody will propose to effect progress by such 
retracing of our steps as ‘‘the extinction of half at 
least of the genera.’’ In ordinary writing one prac- 
tical way of getting round the difficulty is half hinted ° 
at in Mr. Williamson’s last sentence; it is to use the 
family name. Thus we can speak of a Cidarid, a 
Terebratulid, a Phacopid, a Pentacrinid. Or the 
transition may be eased by using the old name with 
some such caveat as sensu lato, e.g. Rhynchonella 
(s. 1.), Antedon (s. 1.). In writing the ‘‘ Guide to the 
Fossil Invertebrate Animals in the British Museum”’ 
the conflict between accuracy and intelligibility was 
settled by adding the older better-known name within 
square brackets after the correct or modern generic 
name, e.g. Dalmanites |[Phacops] caudatus. 
F. A. BATHER. 
Natural History Museum, April 11. 
Electric Emissivity at High Temperatures. 
WE have recently conducted at the National Physical 
Laboratory some experiments on the emission of elec- 
tricity from a number of substances at very high 
temperatures—between 2000° C. and 2500° C. The 
experiments were carried out in a carbon-tube resist- 
ance furnace at atmospheric pressure. Among the 
substances tried were the alkaline earths and a num- 
ber of metals. In every instance the temperature was 
sufficient to vaporise rapidly the substance under 
test, and, under these conditions, very large amounts 
of electricity were emitted. For example, barium 
oxide emitted negative currents of the order of 
4 amperes per sq. cm., while boiling tin gave currents 
of about 2 amperes per sq. cm. No external potential 
was applied in any of the experiments. : 
We hope to publish very shortly a full discussion of 
the results. 
G. Wi. ©: Kaya: 
W. F. HiaGIns. 
National Physical Laboratory, Teddington, 
Middlesex, April 22. 
An Optical Illusion. 
SomE days ago I was reading, the sunrays falling 
aslant on my forehead, it being about five o’clock p.m. 
After having read awhile, the letters showed a vivid 
red, the paper itself retaining its white colour. The 
rays did not fall on either of the two sides of the page. 
When I used my hand to shade the eyes, the letters 
immediately became black again. On removing my 
hand, it took them some seconds to change from 
black through dark red to bright red. I may add that 
I am short-sighted, but was reading without spectacles. 
I should feel obliged if some reader of NATURE 
would give me an explanation of this. 
J. W. Gitay. 
Delft, Holland, April 11. 
