390 THE EVOLUTION THEORY 



But there is not only sexual dimorphism, there is also di- 

 morphism of larvae, e. g. green and brown caterpillars in certain species 

 of hawk-moth (Sphinx), and there are sometimes not only two but 

 three or more forms of a species ; and in all these cases determinants of 

 the differential parts must be represented twice, thrice, or several times 

 in each germ-plasm, in each fertilized ovum, at least in all cases in 

 which the different forms live together on the same area. In 

 discussing mimicry we spoke of species of butterfly which were 

 everywhere alike or nearly so in the male sex, while the females 

 were not only quite different from the males, but differed greatly 

 in many respects among themselves. Three different forms of females 

 of Papilio merope occur in the same region of Cape Colony, each of 

 these resembling a protected model. All three forms have been 

 obtained from the eggs of one female. In this case the female ids of 

 the germ-plasm must be represented by three different sets, one 

 of which, when it is in the majority in the fertilized ovum, gives rise 

 to the Danais-form, the second to the Kavius-iorm, and the third to 

 the Eeheria-iovm of the species. Phylogenetically considered, it is 

 probable that each of these three kinds of ids originated by itself, on 

 a more limited area on which the protected model lived in abundance ; 

 but with a wider distribution the different female ids mingled 

 together, were united through the males into a single germ -plasm, 

 and now occasionally exhibit all three forms on the same area. 

 I doubt whether there is any other theory that can offer an interpre- 

 tation of these facts, and I regard them, therefore, as affording 

 further evidence of the real existence of ids. 



The polymorphism of social insects must be thought of as 

 similarly based in the germ-plasm. 



In bees there are in addition to the males and females the so- 

 called workers, and this can only depend on the existence of special 

 kinds of ids. Those of the workers were originally truly female, but 

 as many of their determinants underwent variations advantageous for 

 the maintenance of the species, they were modified into special 

 ' worker-ids.' I postpone for the present any inquiry into the causes 

 by which these ids come to dominate the ontogeny; obviously it 

 cannot be by the mere fact of being in a majority over the rest 

 of the ids, as I indicated in the case of the butterflies with poly- 

 morphic females. 



In many ants the division of labour goes further still ; there are 

 two kinds of workers in the colon}', the ordinary workers and the 

 so-called ' soldiers/ and in this case the worker-id must have developed 

 in two different directions in the course of phylogeny, and have 



