type,
though why this should have happened does not appear to be clear. At the
same time those specimens which tended to vary in the direction of _A.
echeria_ in places where this species was more abundant than _A.
dominicanus_ were encouraged by natural selection, and under its guiding
hand the form _mima_ eventually arose from _wahlbergi_.

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 effec