40: 



NA TURE 



[August 21, f! 



In some of his recently published experiments, Engelmann 

 found that many of the protoplasmic and unicellular organisms 

 are affected by light, and when the first animals possessed of 

 organs of special sense, viz., the jelly-fish, (Medusae), .ire 

 reached it is found that one particular Medusa ( Tiaropsis 

 polybiadematd) always responds to strong luminous stimulation 

 by going into a spasm or cramp (Romanes). 



But there is a still stronger argument in favour of the powerful 

 action of light on the nerves of the skin in the fact that, as Prof. 

 Heeckel says, "the general conclusion has been reached that in 

 man, and in all other animals, the sense organs as a whole arise 

 in essentially the same way, viz. as parts of the external integu- 

 ment, or epidermis." In fact, that nerves which now see could 

 once but feel. That the highly sensitive optic nerves are but 

 nerves of the skin, whose molecules once could vibrate only in 

 consonance with the large ultra- red waves of heat, whereas now 

 their molecules have become attuned to the shorter waves of the 

 visible part of the spectrum. 



Surely, then, if any one of the nerve-endings of the skin 

 indiscriminately can be specialised for the recognition of light, 

 whether at the margin of the swimming disk in the jelly-fish, at 

 the point of the ray in the star-fish, on the fringe of the mantle 

 in the shell-fish, or on the back in some species of snail, it must 

 be conceded that in the first instance all surface nerves must feel 

 the influence of that agent by which they are to be hereafter 

 exalted. And this has been reduced to a demonstration by Mr. 

 Darwin in his investigations on earthworms, which, although 

 destitute of eyes, are able to distinguish with much rapidity 

 between light and darkness, and as only the anterior extremity 

 of the animal displays this power he concludes that the light 

 affects the anterior nerve-cells immediately, or without the inter- 

 vention of a sense-organ. But a yet more wondrous lesson is to 

 be learned from the steps which Nature takes for the exaltation of 

 a heat-responding nerve into one capable of vibrating in harmony 

 with the shorter waves of light. 



The only external agents available are heat and light, and by 

 these, with such local adaptations as are possible, the conversion 

 must be brought about. 



Seeking again from the lowest organisms the secrets of the 

 highest, it has been found by Engelmann that the simplest 

 creature which responded to luminous stimulation was the pro- 

 toplasmic Englenci viridis ; moreover, that it would only do so 

 if the light were allowed to fall upon the anterior part of the 

 body. Mere there is a pigment spot, but careful experiment 

 showed that this was not the point most sensitive to light, a 

 colourless and transparent area of protoplasm lying in front of 

 it being found to be so. 



From this, the most rudimentary, through the pigmental bodies 

 round the margin of the swimming disk of medusce, and the 

 pigmented ocelli at the tips of the rays in star-fish, to the lowest 

 vermes, in which Prof. Haeckel finds the usual cells sensitive 

 to light separated by a layer 0/ pigment cells from the outer 

 expansion of the optic nerve, we meet with the same arrangement 

 ever progressing upwards, viz., transparency immediately in front 

 of the part to be exalted, and pigment immediately behind it. 

 and are left to infer from the object ultimately attained wrat is 

 the reason of this primary adaptation. 



Nature has made the most of her two factors, by exposing the 

 selected tissue to the continued impinging upon it of the waves 

 of light, while at the same time securing not only the trans- 

 mission through it of the waves of heat, but their constant 

 accumulation behind it, thereby causing the molecular constituents 

 of the protoplasm to be thrown into the highest rates of vibration 

 possibly obtainable with the means at disposal, and undoubtedly 

 more rapid than those of any protoplasm not so situated ; till 

 little by little, by the survival here and there of individuals who 

 had derived some benefit from inherited increase of sensitiveness 

 in the exposed parts, the time arrived when the advantage became 

 permanent in the species, and the foundation was laid in a 

 transparent atom of protoplasm lying in front of a speck of 

 pigment, of those wondrous organs which in teons of ages after- 

 wards were to enable man to look upon the universe and to 

 behold that it was good. 



Such is what light and heat in unison have wrought, and is it 

 to be supposed that their action on the surface nerves is less 

 powerful now than ever ? Is it not more reasonable to think that 

 a larger number of specialised nerves not being an advantage 

 have not been developed, and that though we are unconscious of 

 the power of light upon our bodies, yet that analogy points to the 

 fact that to it, when combined with heat, we owe the highest ex- 

 altation of our keenest sense? 



Recognising thus the effects of simultaneous light and heat 

 when their influence is concentrated by a local peculiarity on a 

 particular part, must it not be evident that in an individual unpro- 

 tected by hair and unscreened by clothes, living beneath the 

 vertical rays of an equatorial sun, the action of these two forces 

 playing through a transparent skin upon the nerve-endings over 

 the entire surface of the body, must be productive of intense, but 

 at the same time disadvantageous, nerve vibrations, and that pre- 

 sumably such individuals as were least subject thereto would be 

 best adapted to the surroundings ? 



V* Nature, therefore, having learned in ages past that pigment 

 placed behind a transparent nerve will exalt its vibrations to the 

 highest pitch, now proceeds upon the converse reasoning, and 

 placing the pigment in front of the endangered nerve reduces its 

 vibrations by so much as the interrupted light would have ex- 

 cited, a quantity which, though apparently trifling, would, when 

 multiplied by the whole area of body-surface, represent a total of 

 nervous action that if continued would soon exhaust the individual 

 and degrade the species. 



Thus it is that man, though so many generations have come 

 and gone since the days of his weaponless struggles with the 

 beasts of the forest, still retains in its full strength that colour of 

 skin which, while it aided him materially in his early escapes, is 

 now continued because it has a more important office to fulfil in 

 warding off the millions of vibrations a second which would 

 otherwise be poured in an uninterrupted stream upon his exposed 

 nervous system. 



Again, the chemical power of light expressed in degree is, 

 according to Professor Bunsen in Berlin, on the 21st of June at 

 12 o'clock, 38°, while at the same place and time on the 2 1st of 

 December it is but 20°, that is, that the difference in the angle at 

 which light strikes the same spot in December and in June causes 

 its chemical effect to be almost doubled. What then must be 

 its potential difference all the year round in the latitude of London 

 and in that of Sierra Leone? 



If, therefore, light be a necessary factor in the development of 

 animal life, and be of sufficient intensity to attain the required 

 end in the northern posil ion of England, it must of necessity be at 

 the equator immensely in excess, all other things being equal of 

 what is needed, and it would be a reasonable expectation that 

 could unclothed man be traced through the parallels of latitude 

 northwards in distinct tribes that never intermingled with those 

 beyond, the colour of the various sections would lessen in direct 

 proportion to their distance from the equator, modified only by 

 such local conditions as materially influenced the effect of light, or 

 the action of light and heat combined. 



And this is forcibly corroborated by the facts put forward in 

 Carpenter's " Physiology," p. 9S5 : " It may be freely admitted 

 that among European colonists settled in hot climates such 

 changes do not present themselves within a few generations ; 

 but in many well-known instances of earlier colonisation they 

 are very clearly manife 



"Thus the wide dispersion of the Jewish nation and their 

 remarkable isolation, maintained by their religious observances 

 from the people among whom they live, render them peculiarly 

 appropriate subjects for such observations, and we accordingly 

 find that the brunette complexion and dark hair which are usually 

 regarded as characteristic of that race are frequently superseded 

 in the Jews of Northern Europe by red or brown hair and fair 

 complexion, whilst the Jews who settled in India some centuries 

 ago have become as dark as the Hindoos around them." 



Finally, there is in a footnote to the same page an extraordi- 

 nary physiological demonstration of the truth of the proposition 

 that skin colour is in direct proportion to light-rays, which is as 

 follows : — ■ 



" A very curious example of change of colour in a negro has 

 been recorded on unquestionable authority. The subject of it 

 was a negro slave in Kentucky, act. forty-five, who was born of 

 black parents, and was himself black until twelve years of age. 

 At that time a portion of the skin an inch wide encircling the 

 cranium just within the edge of the hair gradually changed to 

 white, also the hair occupying that locality ; a white spot next 

 appeared near the inner canthus of the left eye, and from this 

 the white colour gradually extended over the face, trunk, and 

 extremities until it covered the entire surface. The complete 

 change from black to white occupied about ten years, and but for 

 his hair, which was crisp and woolly, no one would have sup- 

 posed at this time that his progenitors had offered any of the 

 characteristics of the negro — his skin presenting the healthy vas- 

 cular appearance of a fair-complexioned European. When he 

 was about twenty-two years of age, however, dark copper- 



