Another instance of experimental breeding involving polymorphism and mimicry in the female sex is that of the African Papilio dardanus, but the case is here complicated by the greater number of female forms (cf. pp. [30]-[33]). The data, too, are far more scanty than in the other two cases, but so far as they go there is nothing to preclude an explanation being eventually arrived at on similar lines[[47]].
And now we may consider briefly the bearing of these experiments on the theory of mimicry. Throughout the work no individuals intermediate between the three well-marked forms of polytes were met with. There is no difference in appearance between the heterozygous and the homozygous mimetic insects, whether they belong to the A or to the H form. The factor X, whether inherited from both parents, or from one only, produces its full effect, and the same is also true of the action of the factor Y. Now the most generally accepted hypothesis as to the formation of these mimetic resemblances supposes that they have been brought about through the gradual operation of natural selection accumulating slight variations.
Professor Poulton, for example, a prominent exponent of this school, considers that the A form of female was first evolved gradually from the M form, and later on the H form came by degrees from the A form. If this be true we ought, by mingling the M germ plasm with the H germ plasm and by subsequently breeding from the insects produced, to get back our series of hypothetical intermediates, or at any rate some of them. We ought as it were to reverse the process by which the evolution of the different forms has taken place. But as is shewn by the experiment of Mr Fryer, which was quoted above, nothing of the sort happens.
From experiments with cultivated plants such as primulas and sweet peas, we have learnt that this discontinuous form of inheritance which occurs in P. polytes is the regular thing. Moreover, we have plenty of historical evidence that the new character which behaves in this way is one that has arisen suddenly without the formation of intermediate steps. The dwarf "Cupid" form of sweet pea, for instance, behaves in heredity towards the normal form as though the difference between them were a difference of a single factor. It is quite certain that the "Cupid" arose as a sudden sport from the normal without the intervention of anything in the way of intermediates. And there is every reason to suppose that the same is true for plenty of other characters involving colour and pattern as well as structure, both in the sweet pea, the primula, and other species. Since the forms of polytes female behave in breeding like the various
forms of sweet pea and primula there is every reason to suppose that they arose in the same way, that is to say, as sudden sports or mutations and not by the gradual accumulation of slight differences.
But if we take this view, which is certainly most consonant with the evidence before us, we must assign to natural selection a different rôle from that which is generally ascribed to it. We cannot suppose that natural selection has played any part in the formation of a mimetic likeness. The likeness turned up suddenly as a sport quite independently of natural selection. But although natural selection may have had nothing to do with its production, it may nevertheless have come into play in connection with the conservation of the new form. If the new form possesses some advantage over the pre-existing one from which it sprang, is it not conceivable that natural selection will come into operation to render it the predominant form? To this question we shall try to find an answer in the next chapter.