Mar. 13, 1873] | 
finding their way back to their homes after having been con- 
veyed from them in such a way as to preclude the possibility of 
their seeing the road by which they travelled, contains, I think, 
the solution of a hitherto perplexing problem. To ascribe this 
power, as is usual, to instinct in the customary sense of the term, 
is to give what Mr. Bain calls ‘‘an illusory explanation of re- 
peating the fact in different language,” and it is manifestly im- 
possible to ascribe it to instinct, as that term is understood in 
the evolution theory of mind. I am glad to see a psychologist 
like Prof. Robertson giving in his adhesion to Mr. Wallace’s 
view. But while in the main accepting it, and arguing forcibly 
in its favour, Prof Robertson hesitates to affirm that it affords an 
explanation of the whole of the facts in question. _Is this failure, 
if failure there be, inherent in the explanation itself, or does it 
lie in our imperfect knowledge of the facts to be explained? 
That there are difficulties cannot be denied. For example, it is 
difficult, to say the least, for the human mind to form the con- 
ception of a sense of smell, so acute, so objective, and furnishing 
sensations so strongly persistent in the ideal, as to enable an 
animal by its means alone, to retrace unerringly long and devious 
roads travelled over but once, and under circumstances rendering 
impossible the co-ordination of sights and smells habitual to the 
animal. In such cases smell must be a much closer second, if 
second at all, to sight, than touch is in man, No blindfolded 
man could perform a like feat by means of unaided touch, nor, 
do I think, could a blind man, though with the blind this sense 
becomes, by the cultivation it receives through a hard necessity, 
greatly more acute than it is in normal cases. But difficulties 
like these are such, I believe, only because of our very limited 
acquaintance with the psychology of the lower animals. One 
of the chief desiderata in mental science is, it seems to me, such 
a psychology, based upon principles generalised according to 
strict inductive methods, from a body of numerous, varied, well- 
authenticated, and scientifically made observations of the 
domestic and other animals. A work of this kind we have not, 
but, I believe, the lines upon which it should be constructed are 
already laid down in Mr. Spencer's truly great work, the 
‘* Principles of Psychology.” When this branch of psychological 
science has been brought into something like parallelism with 
human psychology, difficulties, such as I have hinted at, will, I 
venture to say, be effectually removed, and Mr. Wallace’s ex- 
planation will, as he claims for it, ‘‘cover all the well-authenti- 
cated cases of this kind.” 
In the extended scope claimed for this hypothesis by Prof. 
Robertson; viz., as explanatory of the nature of external per- 
ception in dogs, there appears to be a difficulty raised. The 
most refined and deep-penetrating psychological analysis, of both 
the empiristic and evolution schools, have incontestably estab- 
lished that our mature visual presentations are but symbols of 
the earlier and really genetic presentations acquired through 
touch combined with muscular feeling. Granting, as seems un- 
deniable, that smell in dogs holds, in many respects, a place 
analogous to that of touch in man, would the earliest and the 
genetic presentations of externality in these animals be those 
furnished by smell, with or without the aid of muscular feeling ? 
Before concluding my letter, I should like to offer a remark 
upon the supposed exper imentum crucis of Mr, Wallace’s hypo- 
thesis, suggested by Mr. Bennett. The smell of stale fish would 
undoubtedly interfere with and overpower ordinary smells in the 
human organ. But is it not an anthropomorphical fallacy to 
assume, as Mr. Bennett appears to do, that such would be the 
result in the case of acat? From the almost purely subjective 
and comparatively undeveloped sense of smell possessed by man, 
there appears to me to be no conclusive argument to the highly 
objective and extremely acute sense of smell possessed by certain 
animals. We are not warranted from our own experience in in- 
ferring of a sense, quantitatively, if not qualitatively, so very 
different, that one powerful sensation must necessarily exclude 
fainter sensations of a like order. Normally, vivid sensations of 
a particular order do tend to exclude with more or less complete- 
ness fainter like sensations. But the animal, in the circumstances 
in which it is placed, is as Mr. Wallace shows, in an abnormal 
condition. Its attention is concentrated on the unfamiliar succes- 
sion of smells it is encountering, and under such a stimulus these 
ordinarily fainter sensations may not unreasonably be supposed 
to become unwontedly vivid, and capable of powerfully affecting 
the animal’s consciousness, despite the resistance of what under 
common circumstances would prove an effectual obstacle to their 
conscious presentation. A complete experimentum crucis would 
Tequire that the animal shoul :, during the whole journey, be 
“NATURE 
361 
entirely smell-muffled, and Mr. Bennett’s expedient could not, 
I think, be relied upon to produce this effect. 
Camberwell, March 3 W. H. BREwer 
External Perception in Dogs 
THE following somewhat remarkable instance of a dog finding 
its way back was told me by the owner, who lived 20 or 25 
miles up (and on the left bank of) the river Canuma, in Brazil ; 
a small river just east of the river Madeira. He took the dog 
by boat down the river Canuma and up the Madeira to Borba, a 
small town on the right bank of the latter river; a distance of 
70 or 80 miles round ; and left the dog there. The dog ran 
away from Borba and made its way back to its former home on 
the river Canuma. More exactly, it was making its way back, 
for my informant being out in the wood some little way inland, 
and S.W. of his cottage, fell in with it. It was in bad ~condi- 
tion, having been some weeks—the exact time could not be 
ascertained—in working its way back through the forest, and of 
course had lived by hunting. I cannot give with any exactness 
the distance overland from Borba: perhaps it is less than 25 
miles ; and in this respect the return is not remarkable. It 
seems to me that the dog during its journey by water must have 
had a constant perception of the bearing of its old home ; and 
on the other hand that it made its way back not by any blind 
instinct but by trial and error and by recognition of the cha- 
racter of the forest. F. R. G. S. 
Sight in Dogs 
I THINK Mr. Kingsley rather underrates the exercise of the 
organs of vision by the dog when, in comparing it with the horse, 
he writes,—‘‘ The dog, who has smelt everything, but looked at 
very little.” Now it is true that the dog does not look about 
him when on his travels, in the popular sense, by turning his 
head about, but close observation shows the eyeball in constant 
movement, taking in everything in front and on both sides, 
although, to all appearance, with his head close to the ground, 
his whole attention is concentrated on the reception of external 
impressions through his nose. This 1s particularly noticeable in 
the terrier, which, on meeting you, however intently he may 
seem to be engaged in smelling, gives a quick glance at your face 
without moving his head, or apparently lessening the attention 
he is paying to something else with his nose. Note, also, how 
quickly a dog going down wind sees another a long distance off. 
The horse not only sees and smells acutely but also frequently 
touches any object with his upper lip. : 
In reference to the quotation from ‘ Boswell’s Life of John- 
son,” given by Mr. Nicoll, I may mention that it is well known 
to huntsmen that horses are very prone to kick if led near the 
hounds when a fox is being broken up, the explanation always 
given being that it is the smell of blood which irritates them. 
Faringdon, March 9 J. Hopkins WALTERS 
Selenium 
VITREOUS selenium may be considered a non-conductor of 
electricity. It is only when in a crystalline condition that it be- 
comes a conductor. 
A bar now in my possession, 2°25 x ‘5 x’05 inches, tested 
with an electromotive force of #;th of a Daniell’s cell, gives a 
deflection of 140 divisions on the scale of an ordinary astatic 
mirror galvanometer. The same deflection produced under the 
same conditions through a known resistance, shows the resist- 
ance of the selenium to be 360,000 ohms, By the well-known 
Bridge system the resistance of the same plate of selenium is 
359,500 ohms, the two different tests thus confirming each 
other. 
I have to leave this evening for Valentia to report on the 
electrical condition of the Anglo-American Company’s cable, or 
would write you more fully on the effect of light on the con- 
ductibility of selenium. 
If selenium be expesed to the direct rays of the sun, it gra- 
dually becomes crystalline. May not the explanation of the 
phenomenon be found in this fact ? 
All the bars I have experimented upon have been supplied by 
Mr. H. Bassett, No. 215, Hampstead Road. 
WILLOUGHBY SMITH 
Wharf Road, City Road, March 11 
