In general it may be said that mimetic butterflies are comparatively rare species, but there are exceptions, for instance _Limenitis archippus_ in North America, of which the immune model (_Danaida plexippus_) also occurs in enormous numbers. In another mimicry-category the imitators are often more numerous than the models, namely in the case of the imitation of _dangerous insects_ by harmless species. Bees and wasps are dreaded for their sting, and they are copied by harmless flies of the genera Eristalis and Syrphus, and these mimics often occur in swarms about flowering plants without damage to themselves or to their models; they are feared and are therefore left unmolested. In regard also to the _faithfulness of the copy_ the facts are quite in harmony with the theory, according to which the resemblance must have arisen and increased _by degrees_. We can recognise this in many cases, for even now the mimetic species show very _varying degrees of resemblance_ to their immune model. If we compare, for instance, the many different imitators of _Danaida chrysippus_ we find that, with their brownish-yellow ground-colour, and the position and size, and more or less sharp limitation of their clear marginal spots, they have reached very different degrees of nearness to their model. Or compare the female of _Elymnias undularis_ with its model _Danaida genutia_; there is a general resemblance, but the marking of the Danaida is very roughly imitated in Elymnias. Another fact that bears out the theory of mimicry is, that even when the resemblance in colour-pattern is very great, the _wing-venation_, which is so constant, and so important in determining the systematic position of butterflies, is never affected by the variation. The pursuers of the butterfly have no time to trouble about entomological intricacies.