MIMICRY IN CEYLON BUTTERFLIES. 1974 
acceptance of the hypothesis of those who look to natural seleetion 
as an explanation of polymorphic forms in Lepidoptera :— 
(1) The attribution of selection value to minute variation. 
(2) The absence of transitional forms. 
(3) The frequent absence of mimicry in the male sex. 
(4) The inability to offer an explanation of polymorphism, where 
the polymorphic forms cannot be regarded: as mimics of 
a distasteful species. 
Moreover, the hypothesis assumes that minute variations of all 
sorts can be inherited, a position which at present is lacking in 
experimental proof. 
There is, however, another point of view, which not only avoids 
these difficulties, but is at the same time more in harmony with 'the 
facts of variation and heredity as we are coming to know them. On 
this view natural selection plays no part in the formation of these 
polymorphic forms, but they are regarded as having arisen by sudden 
mutation, and series of transitional forms do not exist because such 
series are not biologically possible. Polymorphic forms may arise 
and may persist, provided that they are not harmful to the species, 
and it is possible to look upon their existence as due to the absence 
of natural selection rather than to the operation of this factor. 
Nevertheless, natural selection, though unconcerned with their 
formation, may play a part in their conservation. To take a definite 
example in illustration. The ‘‘aristolochiz form” of female must 
be supposed to have arisen from the type form as a sudden mutation, 
entirely independently of natural selection. But it is not unlikely 
that the action of natural selection may have aided it in becoming 
established, whether from its resemblance to P. aristolochic, or for 
some other reason. For once in being it is conceivable that even . 
a very slight advantage over the normal form might enable it to 
hold its own with, and even replace, the latter (cf. p. 19, note). But 
whether this is so or not must for the present, in the absence of 
decisive evidence, remain doubtful. Though natural selection may 
operate in the conservation of the polymorphic form, it cannot on this 
view be supposed to play any part in its formation. 
A Suggestion as to the Nature of Polymorphism. 
That polymorphism in a species should so frequently be confined 
to the female sex has long been remarked upon by those who study 
these matters, and the explanation most favoured is that the female, 
burdened as she is with the next generation, is more exposed to the 
action of natural selection and in greater need of some protective 
adaptation. The weak point of such a view is that it does not explain 
why the male is not similarly protected. In. connection with this 
problem recent Mendelian research on sex-limited inheritance is highly 
suggestive. It has been shown that certain types of. inheritance 
receive their simplest explanation on the assumption thatthe 
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