366 
Gardner, he here employs words designed to suggest distinctions 
of organisation between Sigillaria and Lepidodendron, the ex- 
istence of which I altogether deny. 
M. Saporta appears to accept, without demur, statements 
made -by M. Renault respecting Stigmaria ficoides which I 
emphatically reject. These statements are reactionary in the 
highest degree. If true they would c mpel us to cast overboard 
much of the work done during the last half century by Logan, 
Binney, Sir William Dawson, and a host of other observers ; 
work, the reality of which, along with the conclusion; drawn 
from it, was unhesitatingly accepted even by Brongniart himself. 
Such statements, if proven to be true, would involve a rejection 
of all modern views respecting the origin of coal and a return to 
the worthless hypotheses that were believed in half a century ago. 
On this subject I will at present only say that such views 
are absolutely irreconcilable with well-known facts. Should 
these views be allowed to pass unrefuted, as Sir William Dawson 
has properly observed, ‘‘some one will be required to rescue from 
total ruin the results of our labours.”! TI will at present say no 
more respecting these Stigmarian heresies, since I shall have to 
deal with thern more seriously in a work now in hand for the 
Palzontographical Society. 
Mr. Gardner makes one more statement respectinz these 
Lycopodiaceze that is unsupported by any evidence which my 
mich cabinet can supply. He say that ‘‘during growth the 
woody or exogenous zone increased for a certain period, but 
that this was quickly arrested by the absorption or destruction 
in some way of the Cambium layer. The sadseguent increase in 
diameter took place mainly in the cortical system, and to it the 
growth and solidity of the stem was principally due. The 
exogenous element in the oldest known trees is thus seen to 
have been transitory and subordinate, for had it persisted in- 
definitely the continued generation of fresh layers or new rings 
of growth would have produced true Dicotyledonous stems.” 
In the first place we have no evidence whatever of the correct- 
ness of Mr. Gardner's statement. That the vascular axis of 
each of these Lycopodiazeous stems was small in proportion to 
the diameter of its bark is undoubtedly true, and it was equally 
probable that the growth in the thickness of that axis was slow ; 
but I know no facts indicating that such growth ever ceased. 
The diameter of each vascular axis bears about the same pro- 
portion to that of the bark, whether the stems are large or 
small, young or old. Hence we may fairly infer that the cortex 
and vascular cylinders alike continued to grow favi fass'sso long 
as each plant continued to live. Anyhow, I know of no facts 
suggesting a different conclusion. 
Respecting the relation of Calamite to Calamodendron, Mr. 
Gardner says my evidence as to their identity is negative 
rather than positive. If he will honour me with a visit I think 
IT can soon convince him that this is a mistake, and would 
only add that there is little possibility and no probability of 
Mr. Gardner's suggestions being true, viz., that I have ‘‘ sot come 
across an undoubted Calamite,” and that such may be common in 
France though absent from our British deposits. We have them 
by thousands. What I insist upon is that they differ in no 
respect from the so-called Calamodendra, the supposed differ- 
ences being merely due to conditions of preservation. That as 
soon as we get Calamites with any portion of their internal 
organisation preserved, they all prove themselves to be Cala- 
modendra. And that even when their internal orzanisation is not 
preserved the marking on the surface of their thin carbonaceous 
covering itself demonstrates that identity. The volumes of 
MM. Marion and Saporta contain other statements to which, as 
I have informed my friend, I cannot give my assent ; but what 
I now put on record suffices to show the general nature of 
the points on which we disagree. M. Zeiller’s discovery has 
settled the questions of the existence of exogenous Cryptogams 
in the minds of most men—even of several of those who hitherto 
believed in the accuracy of Brongniart’s hypothesis. Patient 
and persevering investigation will, in time, demonstrate which 
of us is right in reference to other debated questions. Mean- 
while the continuance of co-operation and mutual kindly feeling, 
notwithstanding our differences of opinion, must be important 
factors in the attainment of certainty. 
Manchester, July 31 W. C. WILLIAMSON 
Grisebach’s ‘‘ Vegetation of the Earth” 
In No. 823 of your valued paper is an article by Mr. W. 
Botting Hemsley on the new edition of Grisebach’s ‘‘ Vegetation 
? Address to the American Association for the Advancement of Science, 
p- 22, 1883. 
NATURE 
| August 20, 1885 
der Erde,” closing with a reproof to editor and publisher 
for offering the public an old book as new. For my part I 
have to say that it was my strong desire to have a really new 
edition of Grisebach’s classical work, which was no longer to be 
had in the booksellers, by one of our geographical botanists of 
the first rank. ‘his, however, proved unattainable. Seeing I 
was bound by contract to the family of Grisebach, and the son 
of the deceased, Dr. Edward Grisebach, German Consul in 
Milan, insisted on bringing out the ‘‘new” edition himself, all 
entreaties, representations, and explanations were of no avail. 
He declared he would never trust the work of his father to other 
hands and that he felt himself called upon to prepare a new and 
improved edition, TI had therefore but the alternative of seeing 
the work completely disappear or committing the task of a new 
edition to the hands of Dr. E. Grisebach, and I think no one 
will reproach me for choosing the first. At the worst I could 
only look forward to the new edition being a nearly unchanged 
copy of the old work (what in point of tact itis), and this seemed 
to me a far less evil than the complete disappearance of the 
work, an opinion ‘which friendly and competent judges shared 
with me. W. ENGELMANN 
Leipzig, August to 
A Singular Case of Mimicry 
HAVING often read in the pages of NATURE of several cases 
of protection by simulation (or mimicry), I beg to mention one 
which has recently come under my own observation, and which, 
I think, ought to be registered. 
I refer to a small insect which I found in a state of larva, and 
of a white colour, whose back (only) was covered with a layer of 
mo:s, and whose movements in this condition were so natural 
and rapid, that one could immediately perceive that it was the 
natural modus vivend? of the insect. The layer of moss was 
firmly attached to the body, and completely coveredit. I made 
the experiment several times of placing it on its back, feet 
uppermost, on a sheet of paper placed ona table. After a few 
movements the insect, without disturbing the moss, returned to its 
normal position by making certain movements which resembled 
those of an acrobat, who, lying on his back, makes use of his 
hands, and, by a backward somersault, returns to his feet. The 
little creature is so completely disguised by this layer of moss 
that, on placing it on the trunk of a tree covered by the same 
moss, its movements are with difficulty perceived, as the moss in 
movement may easily be confounded with the moss of the tree. 
An insect or larva under these conditions could, only with great 
difficulty, be recognise by its natural enemies (those animals 
which prey on it). 
I send you the specimen to which I refer, the only one I have 
met with, and which may, during the voyage (of thirty days 
more or less), die on the way, or pass through some transforma- 
tion. At all events, you will be able to see the protecting cape, 
and determine the species, larva or insect, which it protects. 
Porto-Alegre, Brazil GRACIANO A, DE AZAMBUJA 
[The larva has apparently passed into the pupa stage during 
the voyage, and has closed the lower side of its protective cover- 
ing with a silken web. Ifthe perfect insect should emerge, we 
will endeavour to ascertain its name.—ED. ] 
Solid Electrolytes 
HAVING been for some months occupied with the electrical 
behaviour of the compounds of copper, silver, and lead with 
tellurium, selenium, and sulphur, I can confirm the observation 
communicated to your pages by Mr. Bidwell as to the behaviour 
of sulphide of copper. He has constructed a primary cell with 
solid sulphides for the electrolytes. The smallness of the electro- 
motive force which he has obtained is entirely due to the close 
proximity of copper and silver in the thermochemical series in 
respect to their heats of combination with sulphur. The theo- 
retical electromotive force should be only ‘o5 volt. 
Let me add to Mr. Bidwell’s observation one of my own. If 
a piece of sulphide of copper is placed between platinum elec- 
trodes, a current of electricity from a battery can be passed 
freely through it, as it is a good conductor. But if after a time 
the battery is removed and the platinum electrodes are connected 
with a galvanometer, a current is observed. The solid sulphide 
between two platinum plates constitutes, therefore, a secondary 
eell or accumulator capable of being charged and discharged. 
SILVANus P, THOMPSON 
Finsbury Technical College, August 17 
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