May 11, 1899] 
UNG T  e 
29 
LEITERS TO THE EDITOR. 
(The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice ts taken of anonymous communications. | 
A Measure of the Intensity of Hereditary 
Transmission. 
THE possessors of certain hereditary characters are unques- 
tionably szd-prolific ; that is, they eventually contribute less 
than their average share to the stock of the future population. 
It may be that they die before the age of marriage, or that they 
are sexually unattractive or unattracted, or that if married 
they are comparatively infertile, or that if married and fertile 
the children are too weakly to live and become parents. It is 
very probable, though I have no trustworthy facts to confirm the 
belief, that persons affected with hereditary insanity are sub- 
prolific because their families, if they have any, are apt to 
contain members who are afflicted in various ways that render 
them less likely than others to live and tomarry. But I do not 
propose to go into the details of this or of any other malady, but 
merely mention it as an illustration of what is meant, when I 
assume that the possessors of some particular characteristic, not 
necessarily a morbid one, and which may be called A, are sub- 
prolific on the average. 
It is a familiar statistical fact that the characteristics of a 
population, taken as a whole, who live under uniform con- 
ditions, change very little during many successive generations. 
So many per million of them are always found to be affected in 
this way, somany per million in that. The birth-rate continues 
the same, so does the death-rate ; similarly as regards the various 
kinds of accident, and also, it may be inferred, as regards each 
form of disease, though it would be difficult to prove this in all 
cases, owing to improvements in diagnosis and nomenclature 
which make the statistics of disease for one period not com- 
parable on strictly equal terms with those of another. It is 
therefore reasonable to discuss what might occur in an ideal 
population, which we will call P, whose characteristics are 
absolutely unchanged during successive generations, and to 
make such small corrections in the results as the conditions 
may require when dealing with real populations. 
P and A being thus defined, it is obvious that the character- 
istic A must be transmitted with exceptional intensity in P. The 
possessors of A leave comparatively few descendants, conse- 
quently those few must be over-richly endowed with A ; other- 
wise the number of the possessors of A would steadily diminish, 
and a P population would be impossible. Wherever a P popu- 
lation occurs, there must exist an inverse relation between the 
intensity with which A is hereditarily transmitted, and the 
prolific faculty of those who possess it. 
This consideration may be of practical importance to actuaries 
in enabling them to estimate more justly than at present, the 
weights to be assigned to different hereditary diseases. It is a 
most difficult and delicate matter to attack this question directly, 
namely by making exhaustive inquiries into the life-history of 
all the near relatives of those who suffer from any serious 
hereditary malady. The difference in the results arrived at by 
different inquirers proves this, and shows the need of some 
second and independent method of investigation. The above 
considerations supply such a method in all cases where the 
frequency of the disease is found to have been approximately 
constant during successive generations of the population taken 
as a whole. 
All that will then be needed, is to find how far those affected 
by the disease in question have been prolific, testing their 
capacity in that way by the number of their adult descendants 
in (say) the second generation, those in the first generation 
indicating little more than their fertility, which, as the children 
may be weakly, is not the same thing as the capacity of the 
parents for contributing to the future population. When the 
descendants in the second generation are neither more nor less 
numerous than the generality, the intensity of the transmission 
of the disease would be the same as that of any neutral quality, 
such as a moderate difference of stature. But if those de- 
scendants were more numerous than the generality, the in- 
tensity of transmission must be less than the average, while if the 
descendants were less numerous, the intensity would be greater. 
It must be clearly understood that this method is of general 
application, and is not intended to be confined to morbid 
characters only. FRANCIS GALTON. 
NO. 1541, VOL. 60] 
Triboluminescence 
THE interesting list of substances mentioned in to-day’s 
review in NATURE of a paper on the subject of the above 
phenomenon, mentioning as substances in which it is con- 
spicuous, cane-sugar, saccharin, hippuric acid, and some still 
more complex organic bodies, might lead one to suppose that 
only substances of an organic nature, in a crystalline state 
exhibit the kind of triboluminescence seen as a flash of light 
when a crystal of such substances is crushed between two 
glasses. But this is not quite exclusively the case, because 
crystals of uranium-nitrate, and perhaps other crystallised salts 
of uranium, emit a very bright greenish-yellow flash when 
pressed to pieces between glass plates. The property seems 
permanent in these crystals, and it is also apparently inde- 
pendent in them of chemical impurities, since any crystallised 
sample of the nitrate, as far as I have tried, shows the light 
flash very strongly, without any apparent loss of brightness by 
long keeping. 
The ruddy light which gleams from under glass or from a 
flint pebble when ground with strong pressure on a grindstone, 
must apparently be a true example of luminescence produced 
by friction, since it is equally visible under water on a 
thoroughly wet, as on a dry, grindstone, where it can hardly 
be supposed to result from high temperature producing actual 
incandescence. But examples of crystals which emit light by 
fracture do not, it appears, present themselves in nearly such 
abundance among mineral substances, as they have now been 
shown in the above-mentioned paper to do in so many cases 
among organic bodies. 
A rather interesting observation of thermoluminescence once 
befel me while making trials of that property in minerals ; 
and as it may afford, perhaps, a ready means of tracing lime 
or calcareous ingredients in certain minerals, it may be useful 
to mention it here, although the mode of excitation used in 
that instance was not by crushing or rubbing, but by heating 
the material. Some fine dust and grains obtained from the 
imterior portion of the mass of the Middlesborough aérolite, 
when the meteorite was first being chemically and micro- 
scopically examined, were found, to my considerable surprise, 
to glow quite distinctly, though not very brightly, with 
yellowish-white light, when sprinkled in the usual way for 
these experiments on a piece of nearly red-heated iron in the 
dark. No such luminescence would, I believe, be evolved 
by that means from pure terrestrial specimens of the pair of 
double silicates of magnesia and iron (olivine and bronzite, 
much less from the moderate sprinkling of nickel iron, and 
perhaps of iron-sulphide found with them), of which the 
stony matter of the meteorite in the main consists. But as 
its stony mass was considered, in the exact chemical analysis 
of the meteorite made by Dr. Flight,! to contain probably, 
besides, an appreciable amount of labradorite or lime-felspar, 
the source of the light may have been this calcareous ingredient 
of the stone, as calciferous rocks and minerals, for the most part, 
shine brightly with various shades from light- to reddish-yellow, 
in the dark, when strongly heated. To whatever chemical 
materials in the stone, however, the light was really due, it 
afforded, at all events, clear proof that no heat of exceedingly 
high temperature can ever have penetrated to the interior of the 
meteorite, even when it was passing at its fall, ina fireball 
through the atmosphere, since the time when it was broken off 
from some parent rock and projected on a celestial course about 
the sun ; for a very moderate degree of heat suffices to expel 
completely from minerals of these luminescent natures all the 
store of thermoluminescent energy which, either originally com- 
municated to them from without by radiation near some exposed 
or denudated surface, or else contracted by them in some more 
mysterious way at great subterranean depths, they more or less 
abundantly possess. A. S. ILTERSCHEL. 
Observatory House, Slough, April 27. 
The New Zealand Godwit (Zimosa nove-selandie). 
THE Maori of New Zealand have an ancient saying or 
proverb, ‘‘ Who can tell where the kuaka (the godwit) has its 
nest?’ No doubt the Maori were well acquainted with the 
singular habit of these birds, in that they leave the shores of 
New Zealand, for a distant land across the seas, about the same 
time that other migratory birds, which have wintered on the 
Pacific Islands located nearer the tropics, are nesting and 
1 Proceedings of the Royal Society, vol. xxxiii., p. 347, February 1882. 
