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TEE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



and to appear where it is absent, and that natu- 

 ral selection is constantly eliminating such tints 

 as are injurious to the species, or preserving and 

 intensifying such as are useful. 



This view is in accordance with the well- 

 known fact of colors which rarely or never ap- 

 pear in the species in a state of nature continu- 

 ally occurring among domesticated animals and 

 cultivated plants; showing us that the capacity 

 to develop color is ever present, so that almost 

 any required tint can be produced which may, 

 under changed conditions, be useful, in however 

 small a degree. 



Let us now see how these principles will en- 

 able us to understand and explain the varied 

 phenomena of color in Nature, taking them in 

 the order of our functional classification of colors 

 (page 527). 



Theory of Protective Colors. — We have seen 

 that obscure or protective tints in their infinitely 

 varied degrees are present in every part of the 

 animal kingdom, whole families or genera being 

 often thus colored. Now, the various brown, 

 earthy, ashy, and other neutral tints, are those 

 which would be most readily produced, because 

 they are due to an irregular mixture of many 

 kinds of rays ; while pure tints require either 

 rays of one kind only, or definite mixtures in 

 proper proportions of two or more kinds of rays. 

 This is well exemplified by the comparative diffi- 

 culty of producing definite pure tints by the mixt- 

 ure of two or more pigments, while a hap-hazard 

 mixture of a number of these will be almost sure 

 to produce browns, olives, or other neutral or 

 dirty colors. An indefinite or irregular absorp- 

 tion of some rays and reflection of others would, 

 therefore, produce obscure tints ; while pure and 

 vivid colors would require a perfectly definite ab- 

 sorption of one portion of the colored rays, leav- 

 ing the remainder to produce the true comple- 

 mentary color. This being the case we may 

 expect these brown tints to occur when the need 

 of protection is very slight or even when it does 

 not exist at all, always supposing that bright 

 colors are not in any way useful to the species. 

 But whenever a pure color is protective, as green 

 in tropical forests, or white among arctic sdows, 

 there is no difficulty in producing it, by natural 

 selection acting on the innumerable slight varia- 

 tions of tint which are ever occurring. Such 

 variations may, as we have seen, be produced in 

 a great variety of ways, either by chemical 

 changes in the secretions or by molecular changes 

 in surface-structure, and may be brought about 

 by change of food, by the photographic action 



of light, or by the normal process of generative 

 variation. Protective colors, therefore, however 

 curious and complex they may be in certain cases, 

 offer no real difficulties. 



Theory of Warning Colors. — These differ 

 greatly from the last class, inasmuch as they 

 present us with a variety of brilliant hues, often 

 of the greatest purity, and combined in striking 

 contrasts and conspicuous patterns. Their use 

 depends upon their boldness and visibility, not 

 on the presence of any one color ; hence we find 

 among these groups some of the most exquisitely- 

 colored objects in Nature. Many of the uneat- 

 able caterpillars are strikingly beautiful ; while 

 the Danaidffi, Heliconidae, and protected groups 

 of Papilionidte, comprise a series of butterflies 

 of the most brilliant and contrasted colors. The 

 bright colors of many of the sea-anemones and 

 sea-slugs will probably be found to be in this 

 sense protective, serving as a warning of their 

 uneatableness. On our theory none of these 

 colors offer any difficulty. Conspicuousness be- 

 ing Jiseful, every variation tending to brighter 

 and purer colors was selected, the result being 

 the beautiful variety and contrast we find. 



But when we come to those groups which gain 

 protection solely by being mistaken for some of 

 these brilliantly colored but uneatable creatures, 

 a difficulty really exists, and to many minds is go 

 great as to be insuperable. It will be well, there- 

 fore, to endeavor to explain how the resemblance 

 in question may have been brought about. The 

 most difficult case, which may be taken as a type 

 of the whole, is that of the genus Leptalis (a group 

 of South American butterflies allied to our com- 

 mon white and yellow kinds), many of the larger 

 species of which are still white or yellow, and 

 which are all eatable by birds and other insectiv- 

 orous creatures. But there are also a number of 

 species of Leptalis which are brilliantly red, yel- 

 low, and black, and which, band for band and 

 spot for spot, resemble some one of the Danaidaa 

 or Heliconidaj which inhabit the same district, 

 and which are nauseous and uneatable. Now, the 

 common objection is, that a slight approach to 

 one of these protected butterflies would be of no 

 use, while a greater sudden variation is not ad- 

 missible on the theory of gradual change by in- 

 definite slight variations. This objection depends 

 almost wholly on the supposition that when the 

 first steps toward mimicry occurred, the South 

 American Danaidse were what they are now, while 

 the ancestors of the Leptalides were like the or- 

 dinary white or yellow Pieridae to which they are 

 allied. But the danaioid butterflies of South 



