vii] THE CASE OF PAPILIO POLYTES 91 



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 hypo- 

 thetical 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 inter- 

 vention 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 



