360 
freshwaters. A larval life so extremely prolonged, 
as Dr. Schmidt points out, is quite unique. The 
rate of growth, moreover, is extraordinarily slow. 
At full size, after about three years’ growth, the 
larve are approximately three inches long, although 
the temperature of the water in which they are 
immersed is comparatively high. In our own waters 
with much lower temperatures most young fishes would 
attain a corresponding length in as many months. 
The extremely slow growth of the larve of the 
European eel is thus an adaptation to the prolonged 
journey. : ; 
It is scarcely possible to understand this unique 
phase in the life cycle of the European eel on the 
hypothesis that the geographical conditions were 
formerly the same as now exist. But if Wegener's 
theory be accepted, the explanation is simple. As 
the coasts slowly receded from one another the 
larval life of what became the European species was 
more and more prolonged by natural selection in 
correspondence with the greater distance to be 
traversed. T. Wemyss FULTON. 
41 Queen’s Road, Aberdeen, 
February 106. 

The Stoat’s Winter Pelage. 
Sir HERBERT MAXWELL’s letter on the above in 
Nature of February 17, p. 220, raises points of great 
interest. Presumably if his glacial explanation be 
correct, stoats taken from the Scottish Highlands to 
the south of England will still become white in the 
winter; whereas stoats brought from the southern 
counties to the north of Britain will remain the 
same colour the year round. Has this ever been 
put to the test ? 
It would be instructive to know whether winter 
coats intermediate in shade between brown and 
cream-white are ever assumed. I ask this from the 
point of view of mutation, which is so much to the 
fore at present. Have, for example, circumpolar 
white animals arisen from coloured ones through 
chance albinos being preserved and increased by 
Mendelian segregation, or have they appeared 
through the selection of paler and paler forms leading 
eventually up to white ? 
Then again, taking Sir Herbert Maxwell’s explana- 
tion as correct, have we not here an example revealing 
how slowly evolution may work? The elimination 
of the arctic winter garb of the stoat in Britain is 
not yet complete, though some thousands of years 
at least must have elapsed since the last ice age. 
One more point: Is the British stoat as regards 
its pelage reverting to the pre-glacial condition, and 
if so, how does this harmonise with the view that 
evolution is irreversible ? JOHN PARKIN. 
The Gill, Brayton, Cumberland. 

Sir HERBERT MAXWELL’S attractive thesis (NATURE, 
February 17, p. 220), that latitude and not winter 
temperature regulates the seasonal change of the 
stoat’s pelage from brown to white, does not meet 
all the facts of the case. Islay is farther north than 
Monreith, and yet in Islay a large proportion of the 
stoats retain their summer colour throughout the 
winter. 
Having made arrangements some time ago to 
obtain specimens of the Islay stoat, regarded by 
Mr. Gerrit Miller as a distinct race, I was struck by 
the fact that individuals killed in December and 
February were in summer coat, This suggested in- 
quiry as to the usual course of events in the island, 
and Mr. Macdonald reported that there white winter 
stoats are rather the exception than the rule: that 
NO. 2785, VOL. I1T| 
NATURE 
of more than 20 stoats he had killed during the winter — 

[Marcu 17, 1923 
of 1921-22, only one was entirely white, although in 
the previous winter the proportion was higher, about 
six being white; but that only in exceptional years 
did the proportion of white individuals attain to 
about half of the total number killed. 
Now the latitude of Edinburgh is not far off that 
of Islay, yet my impression is that here almost all 
the stoats become white in a normal winter. 
These and other facts strengthen the old idea that 
climate is somehow involved in the colour-change, 
which seems also to depend to some extent on the 
condition of the individual animal. 
JAMES RITCHIE, 
The Royal Scottish Museum, 
Edinburgh, February 21. 

Srr HERBERT MAXWELL, in NATURE of February 17, 
p- 220, directed attention to what he considered the 
conditions determining the winter change of colour in 
stoats, and inferred that the tendency to undergo such 
a change is usually the inherited characteristic of some 
particular strain or breed, rather than the outcome of 
any special present local severity of climate. He said 
the effect was most marked in the Highlands of Scot- 
land and diminished regularly as one travelled south, 
until on reaching Cornwall the winter blanching seemed 
almost entirely in abeyance. 
Since his observations appear to be confined to the 
island of Great Britain, Sir Herbert may be interested 
to learn that as a boy at Jersey, about the year 1880, 
I happened to come across a white stoat. This was 
shot by a neighbour, in St. Lawrence valley, and, after 
being stuffed, kept by us for some years. It repre- 
sented a perfect ermine, the fur being pure white 
except for a black tail. I never heard of, or saw, any 
other specimen in Jersey, either white or brown. The 
case seems interesting, for the stoat belonged to a 
breed which must have been free from any extraneous 
admixture, particularly from the north, since that 
remote period in the past, when the French coast (on 
which the Channel Islands are situated) was finally 
separated from Great Britain by the English Channel. 
Further, the climate being mild and uniform, the 
tendency to assume a winter pelage can only have 
resulted from very ancient inheritance. 
R. DE J. F. STRUTHERS. 
Exeter College, Oxford. 

The Subject Index to Periodicals. 
May I add a few words of information to the 
appreciative review of the above publication which 
appeared in NatuRE of February 17, p. 214. Our 
headings are “‘ The Subject Headings used in the 
Dictionary Catalogues of the Library of Congress” 
to which an annual supplement is published. These 
are linked up with the corresponding classes in the 
shelf-classification of that library. The advantage of 
this type of catalogue is that, if properly compiled, 
it combines system and uniformity with the property 
of immediate reference. It is in fact a class catalogue 
in which the headings are arranged in “ index” 
order. Your reviewer’s suggestion that we should 
print a list of the journals indexed in each Class List 
will be certainly adopted when our funds admit of it. 
Our Class Lists for 1915-16 contained such Lists as 
well as Authors’ Indexes, and it was with the utmost 
regret that we were compelled to discontinue these 
features. : 
The following extract from an official letter now 
being circulated widely throughout the British Empire 
may interest some of your readers : 
— a 
