According to Mendelian views, on the other hand, {146} _A. echeria_ arose suddenly from _A. dominicanus_ (or _vice versa_), and similarly _mima_ arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was common and _A. dominicanus_ was rare, its resemblance to the more plentiful distasteful form would give it the advantage over _wahlbergi_ and allow it to establish itself in place of the latter. On the modern Darwinian view natural selection gradually shapes _wahlbergi_ into the _mima_ form owing to the presence of _A. echeria_; on the Mendelian view natural selection merely conserves the _mima_ form when once it has arisen. Now this case of mimicry is one of especial interest, because we have experimental evidence that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one, though at present it is uncertain which is the dominant and which the recessive form. The two have been proved to occur in families bred from the same female without the occurrence of any intermediates, and the fact that the two segregate cleanly is strong evidence in favour of the Mendelian view. On this view the genera _Amauris_ and _Euralia_ contain a similar set of pattern factors, and the conditions, whatever they may be, which bring about mutation in the former lead to the production of a similar mutation in the latter. Of the different forms of _Euralia_ produced in any region that one has the best chance of survival, through the operation of natural selection, which resembles the most plentiful _Amauris_ form. Mimetic resemblance is a true phenomenon, but natural selection plays the part of a conservative, not of a formative agent. [Illustration: PLATE VI.] {147} It is interesting to recall that in earlier years Darwin was inclined to ascribe more importance to "sports" as opposed to continuous minute variation, and to consider that they might play a not inconsiderable part in the formation of new varieties in nature. This view, however, he gave up later, because he thought that the relatively rare sport or mutation would rapidly disappear through the swamping effects of crossing with the more abundant normal form, and so, even though favoured by natural selection, would never succeed in establishing itself. Mendel's discovery has eliminated this difficulty. For suppose that the sport differed from the normal in the loss of a factor and were recessive. When mated with the normal this character would seem to disappear, though, of course, half of the gametes of its progeny would bear it. By continual crossing with normals a small proportion of heterozygotes would eventually be scattered among the population, and as soon as any two of these mated together the recessive sport would appear in one quarter of their offspring.