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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



birds are brilliantly colored, while in the Malay 

 Islands and in the valley of the Amazon the pro- 

 portion does not exceed 33 per cent. Can this 

 distinction be rationally ascribed to any excess 

 of light enjoyed by New Guinea over and above 

 the amount received by the valley of the Ama- 

 zon? Both these respective districts lie under 

 the equator ; both are fruitful, plentifully sup- 

 plied with moisture, well-wooded, and exposed, 

 as far as we can perceive, to very similar mete- 

 orological conditions. But if excess of light can- 

 not be the cause of the superiority of New Guinea 

 over equinoctial Brazil, why should it be put 

 forward to explain the superiority of Brazil as 

 compared with Britain ? Why should the fauna 

 of the Philippine Islands, as is remarked by Mr. 

 Wallace in his invaluable " Glasgow Address," 

 be so rich in species of exceptionally splendid 

 colors ? Can there be in those islands either any 

 excess in the quantity or any peculiarity in the 

 quality of the sunlight ? That there is, no one 

 has yet even attempted to show, and were such 

 the case it would doubtless be traceable in a va- 

 riety of phenomena not limited to the organic 

 world. 



Another important point has been raised by 

 Mr. Bates. He shows that while in many tropi- 

 cal butterflies the males are most splendidly col- 

 ored, the females — in numbers of cases at least — 

 are sombre and insignificant in appearance, so 

 much so that in former times they were often 

 regarded as specifically distinct from their mates. 

 If excess of light, therefore, be the producing 

 cause of the splendor of the tropical Lepidoptera, 

 why should not the effect appear alike in both 

 sexes ? To this argument, however, the reply 

 has been made that in these very species the fe- 

 males are exceedingly sedentary in their habits, 

 remaining generally concealed in shady thick- 

 ets, while the males flutter about in the sun- 

 shine, and, being thus more exposed to light, ex- 

 perience modifications which — transmitted with 

 constant accumulation from one generation to 

 another — have produced the splendor now char- 

 acteristic of their sex. To this question of the 

 relative amount of exposure to light in different 

 stages of existence we shall have to return. 



But the amount — or at least the intensity and 

 clearness — of the sun does not necessarily vary 

 with latitude alone. The air of some countries 

 is more transparent, less obscured by fogs and 

 clouds than that of others. More light evidently 

 reaches the earth's surface on open plains or on 

 table-lands and in deserts than in dense forests 

 and in narrow valleys. Do we find any corre- 



sponding variation in the prevalent hues of the 

 animal population of these respective localities ? 

 Mr. Wallace points out that the most brilliantly- 

 clad birds and insects are dwellers in the forests 

 where the amount of light received is compara- 

 tively scanty. On the other hand, in the deserts, 

 where — as we have already mentioned — light 

 must attain its terrestrial maximum, the prevalent 

 coloration, if not dark, is certainly neither light 

 nor brilliant. As the Rev. H. Tristram remarks, 

 in such regions the smaller mammalia, the birds, 

 the snakes, and lizards, are alike sand-colored, 

 their hues having evidently more reference to 

 concealment than to the influence of an intense 

 illumination. There is indeed, if we wish to come 

 to details, a curious want of harmony in the effects 

 which light is expected to produce. We know 

 that it bleaches in certain cases and darkens in 

 others ; but it is no easy task for us to predict 

 when either of these opposite effects will be mani- 

 fested. Still it is perfectly possible that light might 

 have a bleaching power upon some living organ- 

 isms, and a darkening effect upon others, according 

 to their different molecular structure. There is, 

 for instance, little doubt but that the air of Persia 

 is, as a rule, exceedingly transparent ; the climate 

 is dry, mists and clouds comparatively rare, wood- 

 lands scanty, and the country generally open. 

 We have even heard it stated that there the sat- 

 ellites of Jupiter are occasionally visible with the 

 naked eye. Here, therefore, we have doubtless 

 a case of light in its greatest intensity ; but, ac- 

 cording to Mr. Blanford, Persian specimens are 

 generally paler than their nearest European repre- 

 sentatives. Here, if light be directly concerned, 

 its action must be of a bleaching character ; yet 

 we generally find in mammals, in birds and rep- 

 tiles, as well as in insects, the upper surface, or 

 portion most exposed to the sun, is darker than 

 the under side, or than parts generally kept in 

 the shade. An animal in whom the contrary ar- 

 rangement prevails — such as the common badger 

 — has much of the appearance of a caricature. 

 This darkening of the superior surface of animals 

 is again adduced as an instance of the chromo- 

 genic power of light, a view to which we shall 

 afterward take occasion to revert. 



As regards the comparison between the trop- 

 ical and the extra-tropical fauna? the case may, 

 perhaps, be fairly summed up thus : There are 

 certain cosmopolitan groups whose members, 

 wherever found, are alike devoid of rich or brill- 

 iant coloration ; there are other groups — such as 

 the Ornithoptcra, the Papiliones, the Buprestidje, 

 the Cetoniadre, thetrogons, humming-birds, birds- 



