16 SPOLIA ZEYLANICA. 
process of variation occurred in a portion of the species polytes, and 
those variations which kept pace with P. aristolochie survived and 
eventually formed the ‘‘ aristolochiz form” of female in polytes. 
Considerable powers of discrimination being granted to the hypothe- 
tical enemies of the species, this view minimises the difficulty of the 
selection value of the initial small variations. On the other hand, 
it has serious drawbacks. In the first place, it does not explain, 
any more than does the other view, the continued existence of the 
unchanged polytes living under the same conditions. And secondly, 
it involves the proposition that the ancestral form of the model is 
similar to that of the unaltered male of the mimic—a proposition 
which the consideration of cases where the same species serves as a 
model for more than one mimic at once shows to be untenable. For 
it is obvious that the male of Argynnis hyperbius (P1. IT., fig. 10 A) as 
well as that of Elymnias fraterna (P1.II., fig. 8 A) cannot both be made 
to serve as the ancestral form of Danais plexippus (P1. IT., fig. 9 A). 
We are therefore forced back upon the former view that model 
and mimic were in the beginning widely different, with its attendant 
difficulty of attributing selection value to minute variations. For, 
this they are bound to do who desire to regard natural selection as 
a factor in the formation of these mimetic forms. 
And here we may draw attention to certain other difficulties which 
this view involves. If the mimic has arisen by a series of transitional 
forms, why do these forms never occur in nature? In P. polytes, 
for example, we have a species in which some of the females remain 
unchanged, and we should naturally expect to find transitional 
forms numerous on this view of the formation of the mimetic forms. 
Yet they have never been known to occur, and their absence cannot 
but cast a doubt upon the adequacy of this view as an explanation 
of the facts. 
The difficulty of males so seldom becoming mimetic has already 
been alluded to. There is yet another difficulty with regard to 
polymorphism among females. There are species where the females 
are markedly polymorphic, but cannot be regarded by any stretch 
of imagination as mimicking distasteful forms. No one, I think, 
would venture to match all the different forms of Papilio ormenus 
or P. memnon with appropriate models. ~ And I doubt whether any 
one could find a model for the helice variety of Colias edusa, or the 
valesina form of Argynnis paphia. Yet a scheme which offers an 
explanation of the occurrence of polymorphism among the females of 
Lepidoptera should cover such cases, as well as those in which the 
polymorphic forms bear a resemblance to some distasteful species. 
Apart then from the questions whether the resemblances in many 
cases of mimicry are sufficiently close to be of effective service to the 
mimic, and whether the action of natural selection can be regarded. 
as sufficiently stringent to have brought these resemblances into 
being, there are still the following difficulties in the way of the 
