100 THE EVOLUTION THEORY 



KupKrida' in the tropical iv^-ioiis of the OM World enjoy a certam 

 iuuiiunity on account of their repulsive odour and taste. Here, too, 

 ol>servatit)n ami expcriuuMit have shown that birds, lizards, and 

 predaceous insects leave the butterflies ot" these families unmolested. 

 I need only mention the observation of Trimen chat, imder an acacia 

 much visited by butterflies, on which Mantides — the .so-called 

 prayintj^-insects— caught and devoured large numbers, the wings of an 

 Arrcva or a Danai.^ were never found. These unpalatable butter- 

 flies also possess a motley or at least striking dress, recognizable from 

 afar, and alike on both surfaces; and they also have a slow flight, by 

 which they are readily recognized. They, too, usually assemble in 

 large swarms, and both sexes are alike, or resemble each other 

 closely in colouring, or at least they are both equally conspicuous. 

 But even these cases do not complete the list of butterflies which are 

 protected by their unpalatability ; among the otherwise much- 

 persecuted and therefore palatable Pieridas (Whites) there is an 

 Asiatic genus, Delias, which in all probability belongs to the immune 

 buttei*flies, as their gaily coloured under surface indicates, and among 

 the nocturnal Lepidoptera of ditferent countries and families there are 

 isolated generations which are very gaily and conspicuously coloured, 

 and which are rejected by birds, their unpleasant odour being 

 perceptible at a distance of several feet (Chalcosiida3 and Eusemiida3). 

 The latter no longer fly under cover of night, like their relatives, but 

 have assumed diurnal habits. 



It is to be supposed that the repulsiveness of such ' unpalatable ' 

 butterfies is associated with the food-plant on which the caterpillar 

 lives. Acrid, nauseous, astringent, and actually poisonous substances 

 are produced in many plants, and we shall see later that this is to their 

 own advantage ; these substances pass into the insect, and they do so 

 probably in part unaltered, in part certainly altered, but still they are 

 protective, perhaps even in an increased degree. This is borne out by 

 the fact that many caterpillars of immune buttei-flies live on more or 

 less poisonous plants: the Acra^id^ and Heliconiidas on Passiflores, 

 which contain nauseous substances ; the Danaidas on the poisonous 

 Asclepiada3, which are rich in milky juice or latex ; the Eupla?a3 on 

 the poisonous species of Ficus, the Neotropinas on the Solanacea?, and 

 so on. But there are many genera, rich in species, and distributed 

 over the whole earth, the caterpillars of which live on plants of very 

 various families and characters, and of these the majority of species 

 are palatable, though a few are repulsive in taste and odour, and 

 therefore immune. This is the case in the genus Pa]yiUo. As far 

 back as the sixties Wallace discovered that there were immune 



