This imitation was first accounted for by Mr. Bates in the case of the Heliconidae, a group of showy, slow-flying abundant butterflies possessing "a strong pungent semi-aromatic or medicinal odour which seems to pervade all the juices of their system." It does not follow, of course, that what seems to us a disagreeably smelling fluid should prove distasteful to the palate of a lizard or a bird. But careful observation of the butterflies convinced both Bates and Wallace that they were avoided, or at any rate not pursued, by birds and other creatures; and Belt found that they were rejected by his tame monkey which was very fond of other insects. So their conspicuous wings, with spots and patches of yellow, red, or white upon a black, blue or brown ground, may fairly be considered an example of warning colouration--though Mr. Thayer has with great ingenuity and acumen endeavoured to show that the markings are effective for concealment and that their value as warning marks is doubtful. Now, says Mr. Beddard, "in the same situations as those in which the Heliconias are found there also occur, more rarely, specimens of butterflies minutely resembling the Heliconias, but belonging to a perfectly distinct family--the Pieridae. They belong to the two genera Leptalis and Euterpe, consisting of numerous species, each of which shows a striking likeness to some one particular species of Heliconia. This likeness is not a mark of near affinity; it affects no important character, but only the shape and colouration of the wings." The particular resemblance here described was the origin of the theory of Protective Mimicry, the conditions under which it occurs being, according to Mr. Wallace: 1. That the imitative species occur in the same area and occupy the same station as the imitated. 2. That the imitators are always the more defenceless. 3. That the imitators are also less numerous in individuals. 4. That the imitators differ from the bulk of their allies. 5. That the imitation, however minute, is external and visible only, never extending to internal characters or to such as do not affect the external appearance. There are plenty of examples of this phenomenon, such as the hornet-like moths and bee-like flies of our own country, and many other instances will be found in these pages. One discovered in tropical America by Mr. W.L. Sclater would have much delighted Belt had he come across it. In that region of the world the leaf-cutting ants present a very characteristic appearance as the column proceeds homewards, each ant carrying a piece of leaf held vertically in its jaws; and a homopterous insect has been found that faithfully resembles an ant bearing its burden. The latter is suggested by the thin compressed green body of the insect, and its profile is precisely like that of the jagged edge of the fragment of leaf held over the back of the ant.