PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
82 NATURAL SELECTION HIbuild in holes in banks. Bee-eaters, trogons, motmots, andtoucans all build in holes, and in none is there any differencein the sexes, although they are, without exception, showybirds. Parrots build in holes in trees, and in the majority ofcases they present no marked sexual difference tending toconcealment of the female. Woodpeckers are in the samecategory, since, though the sexes often differ in colour, thefemale is not generally less conspicuous than the male.Wagtails and titmice build concealed nests, and the femalesare nearly as gay as their mates. The female of the prettyAustralian bird, Pardalotus punctatus, is very conspicuouslyspotted on the upper surface, and it builds in a hole in theground. The gay -coloured hang-nests (Icterinae) and theequally brilliant tanagers may be well contrasted; for theformer, concealed in their covered nests, present little or nosexual difference of colour while the open -nested tanagershave the females dull-coloured and sometimes with almostprotective tints. No doubt there are many individualexceptions to the rule here indicated, because many andvarious causes have combined to determine both the colorationand the habits of birds. These have no doubt acted andreacted on each other ;and when conditions have changedone of these characters may often have become modified,while the other, though useless, may continue by hereditarydescent an apparent exception to what otherwise seems avery general rule. The facts presented by the sexual differencesof colour in birds and their mode of nesting are onthe whole in perfect harmony with that law of protectiveadaptation of colour and form, which appears to have checkedto some extent the powerful action of sexual selection, and tohave materially influenced the colouring of female birds, as ithas undoubtedly done that of female insects.Use of the gaudy Colours of many CaterpillarsSince this essay was first published a very curious difficultyhas been cleared up by the application of the general principleof protective colouring. Great numbers of caterpillars are sobrilliantly marked and coloured as to be very conspicuous evenat a considerable distance, and it has been noticed that suchcaterpillars seldom hide themselves. Other species, however,
in PROTECTIVE RESEMBLANCES AMONG ANIMALS 83are green or brown, closely resembling the colours of thesubstances on which they feed, while others again imitatesticks, and stretch themselves out motionless from a twig soas to look like one of its branches. Now, as caterpillars formso large a part of the food of birds, it was not easy to understandwhy any of them should have such bright colours andmarkings as to make them specially visible. Mr. Darwinhad put the case to me as a difficulty from another point ofview, for he had arrived at the conclusion that brilliantcoloration in the animal kingdom is mainly due to sexualselection, and this could not have acted in the case of sexlesslarvae. Applying here the analogy of other insects, I reasonedthat since some caterpillars were evidently protected by theirimitative colouring, and others by their spiny or hairy bodies,the bright colours of the rest must also be in some way usefulto them. I further thought that as some butterflies andmoths were greedily eaten by birds, while others were distastefulto them, and these latter were mostly of conspicuouscolours, so probably these brilliantly coloured caterpillars weredistasteful, and therefore never eaten by birds. Distastefulnessalone would, however, be of little service to caterpillars,because their soft and juicy bodies are so delicate that ifseized and afterwards rejected by a bird, they would almostcertainly be killed. Some constant and easily perceivedsignal was therefore necessary to serve as a warning to birdsnever to touch these uneatable kinds, and a very gaudyand conspicuous colouring with the habit of fully exposingthemselves to view becomes such a signal, being in strongcontrast with the green or brown tints and retiring habitsof the eatable kinds. The subject was brought by mebefore the Entomological Society (see Proceedings, 4th March1867), in order that those members having opportunitiesfor making observations might do so in the followingsummer ;and I also wrote a letter to the Field newspaper,begging that some of its readers would co-operatein making observations on what insects were rejected bybirds, at the same time fully explaining the great interestand scientific importance of the problem. It is a curiousexample of how few of the country readers of that paper areat all interested in questions of simple natural history, that I
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in PROTECTIVE RESEMBLANCES AMONG ANIMALS 83are green or brown, closely resembling the colours of thesubstances on which they feed, while others again imitatesticks, and stretch themselves out motionless from a twig soas to look like one of its branches. Now, as caterpillars formso large a part of the food of birds, it was not easy to understandwhy any of them should have such bright colours andmarkings as to make them specially visible. Mr. Darwinhad put the case to me as a difficulty from another point ofview, for he had arrived at the conclusion that brilliantcoloration in the animal kingdom is mainly due to sexualselection, and this could not have acted in the case of sexlesslarvae. Applying here the analogy of other insects, I reasonedthat since some caterpillars were evidently protected by theirimitative colouring, and others by their spiny or hairy bodies,the bright colours of the rest must also be in some way usefulto them. I further thought that as some butterflies andmoths were greedily eaten by birds, while others were distastefulto them, and these latter were mostly of conspicuouscolours, so probably these brilliantly coloured caterpillars weredistasteful, and therefore never eaten by birds. Distastefulnessalone would, however, be of little service to caterpillars,because their soft and juicy bodies are so delicate that ifseized and afterwards rejected by a bird, they would almostcertainly be killed. Some constant and easily perceivedsignal was therefore necessary to serve as a warning to birdsnever to touch these uneatable kinds, and a very gaudyand conspicuous colouring with the habit of fully exposingthemselves to view becomes such a signal, being in strongcontrast with the green or brown tints and retiring habitsof the eatable kinds. The subject was brought by mebefore the Entomological Society (see Proceedings, 4th March1867), in order that those members having opportunitiesfor making observations might do so in the followingsummer ;and I also wrote a letter to the Field newspaper,begging that some of its readers would co-operatein making observations on what insects were rejected bybirds, at the same time fully explaining the great interestand scientific importance of the problem. It is a curiousexample of how few of the country readers of that paper areat all interested in questions of simple natural history, that I