_Aiptasia mutabilis_ is yellow brown, the tentacles spotted in longitudinal rows, the spots growing smaller towards the tip, thus affording a perfect example of the adaptation of colour to structure. _Anemonia sulcata_ has normally long light yellow pendulous tentacles tipped with rose, but a variety has the column still yellow but the tentacles pale green, tipped with rose. _Bunodes rigidus_ has the column green, with rows of crimson tubercles, the tentacles are flesh-coloured, except the outer row which are pearly; the peristome is green, with brown lips. [Illustration] [22] Allman's Hydroids. Ray. Soc., p. 123. [23] Compare with Hydra above. [24] Allman. Monograph of Tubularian Hydroida. Ray. Soc., p. 135. [25] Allman, _op. cit._, p. 139. [26] Huxley. Oceanic Hydrozoa, pp. 32, 46, 50. [27] Fauna und Flora des Golfes von Neapel. Die Actinien. 1884. CHAPTER X. THE COLOURATION OF INSECTS. In the decoration of insects and birds, nature has exerted all her power; and amongst the wealth of beauty here displayed we ought to find crucial tests of the views herein advocated. It will be necessary, therefore, to enter somewhat into detail, and we shall take butterflies as our chief illustration, because in them we find the richest display of colouring. The decoration of caterpillars will also be treated at some length, partly because of their beauty, and partly because amongst them sexual selection cannot possibly have had any influence. Butterflies are so delicate in structure, so fragile in constitution, so directly affected by changes of environment, that upon their wings we have a record of the changes they have experienced, which gives to them a value of the highest character in the study of biology. In them we can study every variation that geographical distribution can effect; for some species, like the Swallow-tail (_Papilio machaon_) and the Painted Lady (_Cynthia cardui_), are almost universal, and others, like our now extinct Large Copper (_Lycæna dispar_), are excessively local, being confined to a very few square miles. From the arctic regions to the tropics, from the mountain tops to the plains, on the arid deserts and amidst luxuriant vegetation, butterflies are everywhere to be found.