May 25, 1871] 
NATURE 
65 
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LETTERS TO THE EDITOR 
[The Editcr does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Thickness of the Earth’s Crust 
UPON my return to London yesterday, I received the two last 
numbers of NaTuRE (May I1 and 18), in both of which I find 
communications on this subject. In the first of these, by Arch- 
deacon Pratt, that gentleman inserts a quotation from a lecture 
delivered by} me, on January 29, this year, ‘‘ On the Nature of 
the Earth’s Interior” (vide Nature, February 9, 1871), to the 
effect that the recent experimental researches of the eminent 
astronomer and mathematician, M. Delaunay, had destroyed the 
basis upon which the late Mr. Hopkins’s reasonings, as to the 
solidity of the earth’s interior, were founded, and asks the lec- 
turer, z.¢., me, ‘* I wonder why he has taken no notice of my 
letter in reply to M. Delaunay, which was printed in your journal 
for July 1870, six months before the lecture was delivered, and 
which also appeared about the same time in the Philosophical 
Magazineand the Geological Magazine. In this I showed that 
M. Delaunay had evidently misconceived the problem, and that 
Mr. Hopkins’s method is altogether unaffected by his remarks.” 
As Archdeacon Pratt has the candour to admit that “any one 
with an ordinary degree of knowledge of popular astronomy and 
of mechanical action is quite competent to form a good opinion 
on the point in dispute,” I would, in answer to the question he 
puts tome, simply state that, after a careful study of the letter 
he refers to, upon its first appearancein the Philosophical Magazine, 
I purposely avoided referring to it in my lecture, since I failed to 
discover that the author had in it “showed that M. Delaunay 
had evidently misconceived the problem,” or any reasons what- 
soever which could shake my faith in the conclusions of M. 
Delaunay, subsequently confirmed experimentally by M. Cham- 
pagneur. I would also mention that, previous to this lecture, 
I attended the meeting of the Royal Society on the 22nd Decem- 
ber, 1870, expressly to hear a subsequent paper by Archdeacon 
Pratt “On the Constitution of the Solid Crust of the Earth,” on 
which occasion the opinions of Professor Stokes and the experi- 
menial demonstration of Mr, Siemens, as to the untenable nature 
of the author’s conclusions, still further confirmed me in the views 
I put forth subsequently in my lecture. 
It is now superfluous to specify in detail the precise reasons for 
my rejecting the arguments of Archdeacon Pratt, as I have, in a 
great measure at least, been anticipated in so doing by the sub- 
stance of two letters, signed respectively ‘‘ A. M.” and 
““A, H. Green,” which appeared in my absence in the last 
number of NATURE ; to these I may refer in support of my view, 
in which I may also add one of our first English mathematicians 
has concurred ; that M. Delaunay has not changed his will be 
seen from the Proceedings of the Academy of Sciences at Paris, 
March 6, 1871. ‘ 
Having always entertained the highest opinion of the scientific 
labours of the late Mr, Hopkins, I have taken pains to make 
myself acquainted with his writings as far as possible for me; 
but when Archdeacon Pratt states ‘‘ what Mr. Hopkins did may 
be divided into two parts—he first cozceived an idea, which 
was to be the basis of his calculation; and then he made the 
calculation,” I regard the whole pith of the question as embo- 
died in these words, which admit that Mr. Hopkins based his 
elaborate calculations upon az idea, now shown by M. Delaunay 
to be incorrect, whilst the latter gentleman, on the contrary, 
founds his deductions upon premises which he first proves to have 
stood the test of experiment. Where eminent scientific men are 
arrayed on each side of a question of this nature, the remarks 
made in the last paragraph of the archdeacon’s communication 
seem rather out of place, and might be applied with equal force 
in an entirely opposite sense to that intended by their author. 
May 20 DAVID FORBES 
The Geographical Distribution of Insects 
In NATURE (No. 74, p. 435) was a very interesting article on 
geographical distribution by Mr. Wallace, combating some 
recently-urged views of Mr, Murray’s. Mr. Wallace took, as an 
example, the Madeira Islands, and sustained his position upon | 
the numerical statistics furnished by Mr. Wollaston in his books. 
That these conclusions are very different from those arrived at 
by Mr. Wollaston is evident and as a six months’ residence in 


the more remote group of the Canary Islands confirmed to my 
mind Mr. Wollaston’s position, while bringing into relief facts 
utterly incompatible with Mr. Wallace’s, I have ventured to 
publish a few remarks on the question. 
Mr. Murray’s views of the distribution of beetles seem to me 
resolvable into saying that there are two faunas, a tropical (Bra- 
zilian and Africo-Indian) and an extra-tropical one. My own 
slight researches in exotic coleoptera (confined hitherto to the 
Coccinellidz) strongly confirm this ; and a curious instance of 
the connection between the northern and southern extra-tropical 
faunas occurred to me the other day. Zriopis connexa, a rather 
pretty little ladybird, occurs from Hudson’s Bay and Vancouver's 
Island all the way to the Straits of Magellan ; following, of 
course, the line of the Andes. But my object was principally to 
question some of Mr. Wallace’s conclusions with regard to the 
Madeiran fauna. First of all, I was struck by the absence of 
any hypothesis for the origin of the very curious endemic forms 
which form the most important part of the fauna, and which 
most closely unite it to that of the Canaries and Azores. 
These Mr. Wollaston, myself, and apparently Mr. Murray regard 
as affording proof that these islands, or rather groups of islands, 
were once parts of a considerable continent, and I certainly am 
at a loss to see how else they are to be explained ; for though 
Mr. Wallace regards the Madeira islets as possibly formerly con- 
nected, he would, I suppose, be unwilling to extend this to the 
other groups. Mr, Wallace appears to regard Mr. Murray's hy- 
pothesis to be that the Atlantic continent, of which Madeira is 
a remnant, derived its fauna from Europe; but it seems rather 
to be that in the Miocene period (or earlier) there was a similar 
continent, connected indeed with Europe, not deriving its fauna 
from Europe any more than Europe from it. Perhaps the best 
way of answering Mr. Wallace’s view will be to take the case of 
the Canary Islands, whose fauna, resembling the Madeira as it 
does so closely, must have had a similar origin. Here the argu- 
ment from apterous genera fails to a very great extent. Thus 
Carabus is represented by three species, while in S. Spain there 
is one, and in N, Africa only six or seven. Zyorictus has three 
representatives, and here it may be noticed that ants’-nest beetles 
are decidedly not numerous in the islands, so that the “ unusual 
means of distribution” fail on the whole to get them across the 
water. hizotrogus is represented by the closely allied also N. 
African genus Pachydema. Of the very numerous European 
Rhizotrogi only two Sicilian ones are apterous, so that its absence 
in Madeira tells either way. Odiorhynchus is no doubt absent, 
but its place is more than supplied by Ad/antis (20 sp.) and 
Laparocerus (30 sp.). Pimelia again is represented in the 
Canaries by twelve species, and the apterous genera of Hetero- 
mera by more than fifty species, which almost demonstrates the 
necessity of looking for Tenebrionids: in localities where they 
are likely to occur. 
Tarphius it certainly is difficult to conceive carried across by 
winds or waves, seeing that its habits are so retired that it has 
escaped notice till very recently in Europe. Now, however, it 
is beginning to turn up in suitable mountain localities of Anda- 
lusia, Portugal, the Apennines, Sicily, and Algeria ; four species 
are described, and I have seen two others, all agreeing infer se and 
differing in structure from any Atlantic species. Moreover, it 
must have been carried apparently to the Azores as well. Then 
of the peculiar apterous genera quoted, Zhalassophilus, Tor- 
neuma, Scoliocerus, Xenomma, and Mecognathus occur now also 
in Europe, requiring only a collecting-power equal to that of 
Mr. Wollaston for their discovery. There remain as puzzles 
upon the hurricane theory twenty-two blind species in the 
Madeira and the Canaries, and the whole series of Euphorbia- 
infesting species, fifty in number, all winged, and forming for the 
most part special genera. Finally, with regard to the fauna of 
the Azores, the condition of the islands must be taken intoaccount ; 
if the species found round Santa Cruz, Oratava, and Funchal were 
enumerated, about this proportion of European species would be 
found. The best island, Pico, has not been worked, and in the 
others almost all the original vegetation has disappeared. The 
fact that in the scraps (as they literally were) of Euphorbias, Za- 
Phius and Acalles occurred, shows that if any of the pristine- 
flora could be found a fair number of species might be expected. 
Llastrus dolosus may certainly have come from Madagascar by 
the very ingenious route sketched out by Mr. Wallace ; but the 
occurrence of Urania in Madagascar, Brazil, and the West 
Indies suggests a possibly shorter route, even though no Avastrus 
be known as yet to occur in America. 
In conclusion I may state that Iam going to spend a year or 
