Chapter VII: The Senses of Animals (3)

Special experiments seem to show that their limits of vision at the red end of the spectrum coincide approximately with ours; but at the violet end their spectrum is longer than ours. Sir John covered up the visible spectrum, so as to render it dark, and gave the daphnias the option of collecting in this dark space or in the ultra-violet. To human eyes both were alike dark. But not so to the daphnian eye; for while only 14 collected in the covered part, 286 were found in the ultra-violet. The width of the violet visible to man was two inches. Sir John divided the ultra-violet into three spaces of two inches each. Of the 286 daphnias, 261 were in the space nearest the violet, 25 in the next space, and none in the furthest of the three spaces. From which it would seem that, though these little creatures are sensitive to light of higher vibration-period than that which affects the human eye, their limits do not very far exceed ours. We have seen that human beings differ not a little in their limits of violet-susceptibility. We may presume that Sir John Lubbock and those who assisted him in these experiments were normal in this respect. But it is possible that some individuals could have perceived a faint purple where there was darkness to them, and that the majority of the 261 daphnias were collected in the region just beyond the partition between ultra-violet and darkened violet. Still, there is no cause for doubting the general conclusion that daphnias are sensible to ultra-violet rays beyond the limits of human vision.

* * * * *

a., cuticle; b., hypodermis; c., ordinary hair; d., tactile hair; e., cone; f., depressed hair lying over g. cup with rudimentary hair at the base; h., simple cup; i., champagne-cork-like organ of Forel; k., flask-like organ; l., papilla, with a rudimentary hair at the apex.]

Sir John Lubbock has an interesting chapter on problematical organs of sense. In the antennæ of ants and bees there are modified hairs and pits in the integument (at least eight different types, according to Sir John Lubbock), the sensory nature of which is undoubted. But what the sensory nature in each case may be is more or less problematical. Many worms have sense-hairs or bristles of the use of which we are ignorant. Some organs described as tactile or olfactory in the lower invertebrates are so described on a somewhat slender basis of evidence. The sense-value of the bright marginal beads of sea-anemones is unknown. Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs--the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate; by others, as of importance for the equilibration or balancing of the fish.

It will thus be seen that, apart from the possibility of unknown receptive organs as completely hidden from anatomical and microscopic scrutiny as the end-organs of our temperature-sense, there are in the lower animals organs which may be fitted to receive modes of influence to which we human folk are not attuned.

And what are the physical possibilities? We have seen that, through the telæsthetic senses--hearing, vision, and the temperature-sense--we are made aware of the vibrations of distant bodies, the effects of which are borne to us on waves of air or of æther. The limits of hearing with us are between thirty and about forty thousand (or perhaps, in very rare cases, fifty thousand) vibrations per second. But these are by no means the limits of vibrations of the same class. By experiments with sensitive flames,[FN] Lord Rayleigh has detected vibrations of fifty-six thousand per second; and Mr. W. F. Barrett has shown that a sensitive flame two feet long is sensitive to vibrations beyond the limit of his own hearing and that of several of his friends who were present at the experiment. We have some reason to suppose that vibrations too rapid to be audible by man are audible by insects, but not much is known with regard to the exact limits.

The following table shows what is known concerning the æther-vibrations. The figures are those given by Professor Langley:--

Wave-lengths Number of
in thousandths vibrations
Quality of radiations. of a per second Effects on man.
millimetre. in billions.

Limit of photography,
artificial source 0.185 160 none known

Limit of photography,
solar source 0.295 none known

Limit of violet to normal eyes 0.36 833 }
Limit of red to normal eyes 0.81 370 } vision.

Probable inferior limit of
temperature-sensations 9.25[FO] 30 temperature-sense

Longest waves hitherto
recognised with bolometer 30.0 1 none known

From this table it will be seen that, apart from the possible extension of sight beyond human limits, there are possibilities of another sense for the ultra-violet actinic vibrations as different from sight as is the infra-red temperature-sense. Moreover, the temperature-sense for us has no scale; there is nothing corresponding to pitch in sound or colour in sight. It may not be so with lower organisms. Insects, for example, may be sensitive to tones of heat. The bee may enjoy a symphony of solar radiance. I am not saying that it is so; I am merely suggesting possibilities which we have not sufficient knowledge to authoritatively deny. We have no right to impose the limits of human sensation on the entire organic world. Insects may have "permanent possibilities of sensation" denied to us.

Even within our limits there may be, as we have already seen, great and inconceivable differences. We saw that our own colour-sensations are probably due to the blending and overlapping in different proportions of three primitive monochromatic bands, but that in all probability in birds the bands are different, and overlapping is largely prevented. Their colour-phenomena must be inconceivably different from ours. And what shall we say of the colour-vision of invertebrates? Are we justified in supposing that for them, as for us, R., G., and V. are the unstable explosives, and that they are present in the same proportions as with us? If not, their colour-world cannot be the same as ours. Of the same order it probably is. And all that we can hope to do is to show, as has been shown, that colours which differently affect us affect them also differently.

* * * * *

In conclusion, we may return to the point from which we set out. The organism is fitted to respond to certain influences of the external world. The organs for the reception of these influences are the sense-organs. When they are stimulated waves of change are transmitted inwards to the great nerve-centres; they are there co-ordinated, and issue thence to muscles or glands. Thus the organism is fitted to respond to the influences from without. The activities of organisms are in response to stimulation.

We have seen that the cells of the organic tissues are like little packets of explosives, and that the changes which occur in the organism may be likened to their explosion and the setting free of the energy stored up in them. The end-organs of the special senses may be regarded as charged with explosives of extreme sensitiveness. Some are fired by a touch; the molecular vibrations of sapid or odorous particles explode others; yet others are fired by the coarser vibrations of sound; others, once more, by the energy of the ætherial waves. The visual purple is a highly unstable chemical compound of this kind; expose it for a moment to light, and it topples over to a new molecular arrangement, the colour being at the same time discharged. If the retina has been removed from the body, this is all that happens. But if (in the frog) it be replaced on the choroid layer from which it has been stripped, the visual purple is reformed. The explosive is thus reconstructed and the sensibility is restored. Thus, as fast as the explosives are fired off by sense-stimuli, so fast in normal life are they reconstituted and the sensibility restored. Meanwhile the explosion at the end-organs has fired the train of explosives in the nerve, and created molecular explosive disturbances in the brain. Thence the explosive waves pass down other nerves to muscles or glands, and, giving rise therein to further explosions, take effect in the activities of the organism.

We shall have to consider these activities hereafter. We must now turn to the psychical or mental accompaniments of the explosive disturbances in the brain or other aggregated mass of nerve-cells.

NOTES

[EO] See abstract in _Nature_, vol. xxxiv. p. 515.

[EP] See _Nature_, vol. xxxvii. p. 557.

[EQ] "Sense-Organs and Perception of Fishes:" Journal of Marine
Biological Association, New Series, vol. i. No. 3, p. 225.

[ER] _Nature_, vol. xlii. p. 201.

[ES] _Nature_, vol. xxxvi. p. 273.

[ET] Journal of Marine Biological Association, New Series, vol. i. No.
3, p. 235.

[EU] Mr. S. Klein mentions a similar fact in connection with _Bombyx
quercus_ (_Nature_, vol. xxxv. p. 282).

[EV] Journal of Marine Biological Association, New Series, vol. i. No.
2, p. 211.

[EW] A friend of mine informs me that his limit is about 17,500 per
second, 20,000 being quite inaudible.

[EX] Journal of Marine Biological Association, New Series, vol. i. No.
3, p. 251.

[EY] Of course, anglers will say that what may be true for pollack and
other coarse and vulgar sea-fish does not apply to King Salmon or
Prince Trout.

[EZ] "Senses of Animals," p. 117.

[FA] See a very interesting and lucid paper by Professor Crum Brown,
whose name is intimately connected with this subject, in _Nature_,
vol. xl. p. 449.

[FB] It is interesting to note that in the blind-fish (_Amblyopsis
spelæus_) the semicircular canals are, according to Wyman,
unusually large.

[FC] The dampers must, of course, be lifted by depressing the loud
pedal.

[FD] "Special Physiology," p. 636.

[FE] A band and not a line, because R. is unstable to the impact of a
considerable range of light-vibrations.

[FF] Mr. Chattock has kindly supplied me with the following note:--

"Readings at the violet end were taken at the extremity of the
lavender rays, at the point where the faint band of lavender light
seemed to end off about half-way across the field of view (the
cross-wires being invisible).

"At the red end the cross-wires were always visible, and were in
each case set to the point where the top horizontal edge of the
spectrum lost its definition.

"Other things equal, the 'red' readings should be more reliable than
the violet, therefore, from the greater definiteness of the point
observed, and the means of observing it. But against this has to
be set off the fact that the extreme violet rays were spread out
by the prism used more than eight times as much as the red rays.

"In any case, the wide differences observed in the 'red' readings
are much greater than could have been due to misunderstanding or
careless observation--as shown by setting the instrument to
maximum and minimum readings, and noting the very obvious
difference between them apparent to a normal eye. The same
conclusion is rather borne out by the closer (average) agreement
between the two eyes of the same individual than between those of
different persons.

"The source of light was the central portion of an ordinary Argand
burner."

[FG] The variations above indicated throw light on a fact to which Lord
Rayleigh has directed attention. The yellow of the spectrum may be
matched by a blending of spectral red and spectral green; but the
proportions in which these spectral colours must be mixed differ
for different individuals. The complementary colours for different
individuals are also not precisely the same.

[FH] "Colour-Vision and Colour-Blindness," R. Brudenell Carter
(_Nature_, vol. xlii. p. 56).

[FI] Journal of Marine Biological Association, New Series, vol. i. Nos.
2 and 3. His experiments with regard to the colour-sense in fishes
gave, for the most part, negative results.

[FJ] We must remember how largely the antennæ are used when an insect
is finding its way about. Watch, for example, a wasp as it climbs
over your plate. If the antennæ be removed, it seems to stumble
about blindly. The antennæ seem almost to take the place of eyes
at close quarters.

[FK] "Senses of Animals," p. 194.

[FL] See _Nature_, vol. xli. p. 407.

[FM] Chap. x. p. 202.

[FN] The observations are not yet published, and I have to thank Lord
Rayleigh for his courtesy in allowing me to make use of this fact.

[FO] Professor Langley finds that the maximum effect with a radiating
source
at 170° C. is at about 5.0 thousandths of a millimetre wave-length.
" 100° C. " " 7.5 " " "
" 0° C. " " 11.0 " " "

We are sensitive to radiations from a body at 100° C. But when the
temperature falls below the normal temperature of the body we are
not sensitive to heat-vibrations, but to loss of heat from the
surface exposed. The limit of sensibility to heat-vibrations,
therefore, probably lies between 7.5 and 11 thousandths of a
millimetre. I have taken about 9.25 as the limit.