
iil 
Dec. 1, 1870] 
NATURE 
85 

viewed from the upper surface. The cerebrum presents 
the following parts in successive order :—1, the medulla 
oblongata; 2, the cerebellum ; 3, the lobus opticus, 
with its median furrow; 4, the lobus ventriculi terti 
(thalami optici of authors); 5, the lobi hemispheerici, 
each of which terminates anteriorly in a knob constituting 
the tuberculum olfactorium. On the under surface of the 
brain there appear successively from before backwards :— 
(1) the bases of the lobi hemispheerici ; (2) the chiasma 
of the optic nerves, which last proceed from (3) the lobus 
opticus, and between which is situated (4) the hypophysis 
cerebri, and behind this (5) the base of the medulla oblon- 
gata. M. Stieda then gives a-full description of these parts, 
and of the variouscerebral nerves in the frog. To this suc- 
ceeds avery good general view or 7éswaé of the anatomy of 
the brain in mammals. We may draw attention to some re- 
marks made in the section where a comparison is made be- 
tween the brain of man and that of the several classes of 
Vertebrata. It maybe premised that little difficulty is experi- 
enced in discovering the homologous parts of the central 
nervous system of man andthe morehighly organised mam- 
mals. {In the birds, however, there are several parts that are 
difficult to decipher ; whilst in Amphibia, and still more in 
fishes, the nature of the several parts has given rise to 
much discrepancy of opinion between different observers, 
Dr. Stieda refers to his former work for the brain of fishes. 
In regard to Amphibia and reptiles, he considers that the 
lobi hemisphzerici, or anterior lobes, being hollow, and 
containing a ventricle, are clearly the analogues of the 
cerebral hemispheres of man. The azygous portion of the 
central cavity, between the posterior parts of the hemi- 
spherical lobes (or ventriculus.communis) in :the frog, is 
theindication of the primordial single cavity of the first cere- 
bral vesicle, and consequently establishes the transitional 
stage between the osseous fishes and the higher Vertebrata. 
The succeeding segment constituting the lobus -ventriculi 
tertii, (or Zwischenhirn) corresponds in its upper part to the 
thalami optici; in its lower to the tuber cinereum and 
lamina cinerea. The third segment, or lobus opticus, 
agrees exactly with that of fishes, both in its external and 
internal relations, whilst reptiles exhibit the inter- 
mediate type between fishes and birds. Of the nature of 
the cerebellum there can be no doubt. In regard to birds, 
he observes, that the great club-like segment of the 
cerebrum of birds corresponds to the hemisphere of man, 
the bodies enclosed in them to the corpora striata, the 
radiated septum to the septum pellucidum. Heconsiders 
the existence of parts analogous to the corpus callosum 
and fornix of man to be doubtful. The succeeding seg- 
ment corresponds to the optic thalami; the large spheroidal 
body of the lobus opticus to the corpora quadrigemina. 
Two plates accompany the treatise, which are devoted to 
the histology of the parts described. else lee 



LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
The Difficulties of Natural Selection 
As Mr. Bennett complains that I have charged him with 
errors he has not committed (which I should much regret to have 
done), I must ask permission to justify my statements by a re- 
ference to his own words. 
1. Mr. Bennett says that he is unable to discover where he has 
led his readers to understand that there is only one completely 
mimicking species of Zeféalis, I will therefore show him where 
he has done so. In the third column of his article (p. 31) he 
says: ‘“‘Another South American genus of Lepidoptera, the 
Leptalis, belongs structurally to an entirely different class, the 
Pievide, and the majority of its species differ correspondingly 
from the //eliconide in their size, shape, colour, and manner of 
flying, being nearly pure white. There is, however, one par- 
ticular species of Leftalis, which departs widely in external facies 

from alt its allies, and so closely resembles a species of /¢homia 
as to deceive,” &c. &c. Then comes the argument and the 
mathematical calculations always referring to ‘the Zepéalis,” 
and it is at the end of this, at the bottom of the next column, 
that we have the following passage (of which Mr. Bennett in his 
reply has only quoted a line and a half): ‘‘ For supposing the 
chance isreduced from one in ten million to one in ten thousand, 
and it is said that the world has existed quite long enough to 
give a fair chance of this having occurred once, it is not a soli- 
tary instance that we have. Mr. Bates states that in a com- 
paratively small area several distinct instances of such perfect 
mimicry occur, Mr, Wallace has a store in the Malay Archipe- 
,lago, Mr. Trimen records several ef- wonderful completeness in 
South Africa,” &c. Now, as thereisnot a word here about ether 
species of Zeffalis, but only about other cases of mimicry, as 
Lepialis is unknown in Africa or the East, as mimicry occurs in 
other genera and families of Lepidoptera, and other orders of 
insects, and as Mr. Bennett has himself stated, that the ‘‘ ove 
particular species of Leptalis departs widely in external facies 
from ad/ its allies,” I think it will be admitted that I was 
justified in asserting that Mr, Bennett’s readers would be ‘‘led 
to understand,” that there was only one species of completely 
mimicking Zef/alis. If I was not so justified I confess my 
ignorance of the English language, and beg Mr. Bennett’s 
pardon. 
2. I leave your readers to judge for themselves whether the 
fact of a Leftalis having twenty offspring does or does not affect 
the mathematical argument as set forth by Mr. Bennett ; but 
when, in answer to my statement, that the right variation has, 
by the hypothesis, a greater chance of surviving than the rest, he 
asks: ‘* By what hypothesis? ‘he hypothesis that these small 
variations are useful to the individual, the very hypothesis against 
which I am.contending as unproved,”—I must protest against his 
denying his own words. For, at p. 31, col. 1, he says: ‘*‘ The 
next step in my argument is, that the smallest change in the 
direction of the /thomza which we can conceive, on any hypo- 
thesis, to be beneficial to the Zeffa/zs is, at the very lowest, one- 
fiftieth of the change required to produce perfect resemblance ;”’ 
and six lines farther on, ‘‘ For the sake of argument, however, 
I will suppose that a:change to the extent of one-fiftieth 2s deve- 
ficial,” and then-comes the calculation. Again, I must acknow- 
ledge my ignorance of the meaning of words if Mr. Bennett 
does not here directly contradict himself. I never said the 
hypothesis was proved, but only that Mr. Bennett’s argument, 
founded:on it, was unsound, and forthe sake of the argument 
he had admitted the hypothesis. 
Mr. Bennett goes on to say: ‘‘ The new factor, of which I 
take no account, is, again, entirely dependent on the admission 
of the natural selectionist premiss.” This new factor is the 
principle of Aeredity. As he acknowledges that he takes no 
account of it, we must presume that he denies its existence ; and 
as the whole. of Mr. Darwin’s theories and my own fall to the 
ground without it, he might have spared himself the trouble of 
his “mathematical demonstration.” 
3. Ido not consider, as Mr. Bennett seems to do, that the 
distinction between ‘‘ protective resemblance” and ‘mimicry ” 
is asubtle one. Anyone who reads his paragraph on this sub- 
ject (p. 32, col. 2) will, I think, be under-the impression, as I 
was, that he alluded to mimicry, or mimetism, properly so called, 
as being strongly developed in birds, It seems, however, that 
he means only protective resemblance ; but this, I believe, to be 
equally common among the-very lowest forms of life. Trans- 
parency, for example, is a great protection to aquatic animals, 
and it is very prevalent in low organisms. Fishes are all, or 
almost all, protectively coloured, by the back being dark and the 
belly light, so that, whether looked at from above on the dark 
background, or from below on the light one, they are equally 
difficult to see. In many fishes, too, we havea specific protective 
resemblance as perfect as in any birds (see ‘‘ Contributions to the 
Theory of Natural Selection,” p. 55), and this is as much opposed 
to Mr. Bennett’s theory as the absence of tiue mimicry in birds 
and mammals. 
4. Mr. Bennett says, I have “brought no evidence to show 
that extremely small variations afford any immunity from the 
attacks of enemies,”—but this was quite unnecessary, because I 
show that the variations which continually occur in insects are 
by no means ‘‘extremely small.” Tezalso sas that I “give no 
explanation of the tendency of the Zeféa/?, referved to by Mr. 
Bates, to produce naturally varieties of a nature to resemble 
Ithomie.” But Mr. Bates introduces this remark with—‘‘ It. 
would seem as if” and though I think that the fact may bé so, 
