Though the cases just quoted are only very problematically mimetic, N. America has yet several examples of resemblance between distantly related forms as close as any that occur in the tropics. In this region are found two species of the genus _Danais_--_D. archippus_ occurring all over the United States and reaching up northwards into Canada, _D. berenice_ found in the South-eastern States, _e.g._ in Florida, where it is said to be more abundant than _archippus_. {49} _D. archippus_ (Pl. XVI, fig. 8) is very similar to the oriental _D. plexippus_ (Pl. IV, fig. 2), from which perhaps its most notable difference lies in the extent and arrangement of the white spots near the tip of the fore wing. _D. berenice_ is not unlike _archippus_ in its general colour scheme but is smaller and darker (Pl. XVI, fig. 9). We have already had occasion to mention the common Nymphaline, _Limenitis arthemis_ (Pl. XVI, fig. 4) which is found in Canada and the Northeastern States. Widely spread over N. America is a close ally of this species, _L. archippus_, which, though so similar in structure and habits, is very different in external appearance. As appears from Pl. XVI, fig. 6, _L. archippus_ is remarkably like the Danaid which bears the same specific name. In the Southern States _L. archippus_ is replaced by a form slightly different in details of pattern and distinctly darker, _L. floridensis_ (= _eros_) (Pl. XVI, fig. 7). In Florida occurs also the darker N. American Danaid, _D. berenice_, to which the colour of _L. floridensis_ approximates more than to _D. archippus_, and it is of interest that although the last named is also found in this locality it is said to be much less abundant than _D. berenice_. Nevertheless it appears to be true that the range of _L. floridensis_ is much more extensive than that of its model; in other words, that there are considerable regions where _L. floridensis_ and _D. archippus_ coexist, and from which _L. archippus_ and _D. berenice_ are wanting. * * * * * {50} CHAPTER V SOME CRITICISMS The facts related in the last two chapters are sufficient to make it clear that these remarkable resemblances between species belonging as a rule to widely different groups constitute a real phenomenon, and as such demand an explanation. One explanation, that in terms of the theory of mimicry, has already been outlined, and we may now turn to consider it in more detail. Some years ago Wallace[24], combating the suggestion that these instances of resemblance might be mere coincidences, laid down five conditions which he stated were applicable to all such cases, and rendered utterly inadequate any explanation other than in terms of natural selection. These five conditions are of historical interest and may also serve as a peg for sundry criticisms in connection with the mimicry theory. They are as follows: