-instances as that adduced by your correspondent. 
Ret awe, . 
3] 
LETTERS TO THE EDITOR 
[Zz he Editor does not hold himself responsible for opinions expressed 
by his correspondents, No notice is taken of anonymous 
communications. } 
Permanent Variation of Colour in Fish 
A QUESTION of some interest is raised by a letter published 
by Mr. Saville Kent, in NATURE, vol. viii. p. 25. It is stated that 
a Plaice, now in the Brighton Aquarium, has ‘‘the posterior 
half of its under surface, usually white, coloured and spotted as 
brilliantly as the upper one; the line of demarcation between 
these two colours again, though sinuous, is most abrupt,” and 
the writer proceeds to say that, on the Darwinian theory, this 
may be considered as a remarkable instance of reversion—“‘ the 
Pleuronectidze being derived from ancestors originally possessing 
bilateral symmetry, and an equal degree of coloration on each 
side. 
First, as to the fact :—Examples of such colouring among the 
Pleuronectide are not very uncommon, and they occur most 
frequently in the Flounder (Pleuronectes flesus) and Plaice 
(P. platessa). Sometimes it is the upper surface which is thus 
affected—more or less of it being purely white. In a specimen 
now before me the colouring of the upper surface occurs upon 
the under one in numerous blotches of various sizes, and this 
mode of distribution is not uncommon. In every instance that 
I have heard ef, the line or lines of demarcation, when they 
exist, are such as your correspondent describes, but, in extreme 
cases, no such line is present—the whole of one surface having 
unitormly assumed the colouring of the other. Such abnormal 
colouring may occur either upon the upper or lower surfaces ; 
the fish in the former case being entirely white, and in the latter 
entirely brown. 
The rationale offered by your correspondent, although en- 
ing, is not unopen to criticism. For nothing can be more 
evident to Darwinists than that the colouring of the Pleuronectide 
has been acquired because of its protective adaptation to their 
peculiar form and habits. But it is difficult to see how such 
colouring could have conferred protection upon their free- 
swimming ancestors, so that, unless we make the highly anti- 
Darwinian supposition that the common progenitor was coloured 
in anticipation of the habits to be contracted by its offspring, 
there is only one hypothesis open to us, viz., that the unmodified 
progenitor adopted, through natural selection, the habit of lying 
on its side Jecause of its original sandy colour. As this view, 
however, will be rejected by all who know how much easier 
colour is to modify than habit or structure, we are compelled to 
adopt the supposition, as being the most probable, that the 
coloration of the P/euronectide is the result and not the cause of 
their form, and has, therefore, been acquired during the process 
_ of their flattening. 
Although; however, we cannot, without gratuitous supposition, 
imagine that the unmodified ancestor of the group in question 
was coloured exactly like his progeny, there is still one other 
hypothesis by which atavism might be called in to explain such 
Whatever 
may have been the original cause of the flattening taking place, 
it is not likely that the initial variations (whether these were 
sudden and considerable, or gradual and slight), presented nearly 
so great a modification as that which we now observe. During 
these initial stages the partially modified individuals may have 
lain indifferently on either side, and so have acquired protective 
colouring on both. As the flattening, however, proceeded (from 
whatever cause), and the bones ‘of the skull, etc., became more 
and more contorted, the new exigencies of the case might have 
caused the left side to be more and more usedas a ventral 
surface, until its colouring, being of no further use, was allowed 
gradually to disappear. Upon this view the deviations from the 
normal colouring which now occur would be reversions, not to 
the bilaterally-symmetrical ancestors of the flat-fishes, but to their 
partially modified offspring. And, if this view were tenable, it 
might throw some light upon the otherwise inexplicable fact that 
some species of Pleuronectide are normally reversed—z.e. the left 
side instead of the right, constituting the upper surface—while 
in both kind of species individuals often occur which are reversed 
with reference to their specific type. 
As however, this explanation is rather far-fetched, and, more- 
over, fails to account for the appearance of the partly white 
and the wholly white specimens above mentioned, it is best, I 
think, altogether to abandon the reversion theory, 
Another, and, to my mind, a more probable oneis open to us. 
NATURE 
IOI 
Accepting the occurrence of abnormally reversed fish as an un- 
explained fact, we might, @ Ariorv7, expect that a cross between a 
normal and a reversed individual of the same species might pre- 
sent the appearance described in your correspondent’s letter—the 
abrupt, though sinuous line of demarcation between the two 
colours, which always attends the occurrence of this variation, 
being precisely analogous to that which obtains in higher 
animals when piebald. Moreover, the abnormal coloration being 
of most frequent occurrence in the Flounder and Plaice—fish 
which are also the most frequently reversed—and the occasional 
appearance of the entirely white and entirely brown varieties, are 
just the facts we should anticipate were this explanation the 
correct one. Of course it may be objected that abnormal 
colouring is not of nearly so frequent occurrence as abnormal 
reversal, but when we remember how utterly ignorant we are 
regarding the causes which determine reversal in the Pleuronec- 
tide, and the blending or non-blending of colours in all animals 
when crossed, we should not lay too much stress upon this ob- 
jection. 
’ The truth or falsehood of this explanation would admit of 
easy experimental test on the part of the Brighton Aquarium 
authorities. Should they, however, undertake such, they must 
not rest satisfied with mere simple crosses, however numerous, 
but also try various complex and reciprocal ones. The piebald 
fish they possess should also be crossed with several normal and 
reversed Plaice. Should all their experiments prove unsuccessful, 
they would still be interesting as tending to throw us back upon 
the only remaining explanation, viz. that all these instances of 
abnormal coloration are independent sports, and so affording us 
by far the most striking of the many examples in the animal 
kingdom of the tendency towards bilateral symmetry which ab- 
normal colouring frequently presents. 
Dunskaith, Ross-shire, May 15 GEORGE J, ROMANES 
Venomous Caterpillars 
THE concluding words of Mr. H. S. Wilson’s letter in your 
last number only reiterate the truth of a fact. Nearly all British 
entomologists who have collected Zepidopiera must have had 
painful experience of the irritation caused by the hairs of some 
one or other of our Bombyces that have very hairy larvae. or- 
thesia chrysorrhea is the greatest delinquent in this respect ; and 
some years since I suffered intense agony after collecting the 
pup of this species. The hairs of the caterpillar are woven 
into the cocoon and the web surrounding it, and I recommend 
anyone in search of a counter-irritant to rub his face and neck 
with his hands after collecting these pupz. The result, although 
painful, will be edifying and admonitory. The hairs have no 
effect upon the harder skin of the palm of the hand and fingers ; 
and I believe (with most entomologists) that their action is purely 
mechanical, 7.e. they pierce the tender skin in multitudes. A 
precisely similar, though less severe, effect is caused by the hairs 
of some Boraginaceous plants, ¢.g. Echium vulgare, On the Con- 
tinent the extreme irritation caused by the hairs of Cnethocampa 
procéssionea is well known ; and the introduction of a brood of 
these larvce into a drawing-room would probably be followed by 
effects similar to those caused by the king’s ‘“‘great flea” in 
Faust. 
At present I consider that the existence of caterpillars actually 
venomous (¢.¢. with a poison-gland at the base of each hair) re- 
quires confirmation. There are some pachydermatous individuals 
upon whom the hairs of Bombyces have litde or no effect. I 
am unhappily not one of those, but my mental hide repels the 
insidious attacks of romancers in Natural History. 
Lewisham, May 16 Rosert McLACHLAN 
BETWEEN the years 1857 and 1862 when stationed at Be- 
lozi, the capital of British Honduras, I made the acquaintance 
of a so-called venomous caterpillar, which was held in very 
great dread by the natives, who averred that ‘‘its die always 
produced fever.” 
Knowing their super-titious habits, and that, as far as my 
knowledge of natural history went, there did not exist a cater- 
pillar capable of producing a wound of any kind by diting, I 
resolved to test the truth of the assertion, Accordingly, and to 
the intense horror of the bystanders, I took one in my hand from 
a tree that was literally covered with them, It was about 
14 in, long, by 2 in. thick, of a blue-grey tint, and in addition 
to the fine long hairs which clothed it, was armed with clusters 
of short spines. These clusters were formed nto rows 
