PROTECTIVE EESEMBLANCE3— DIMORPHISM, ETC. 49 



302 for an example of a dimorphic male among 

 spiders). Its meaning is obscure, but one of the two 

 forms is generally much rarer than the other, and 

 probably the older. The facts seem to point towards 

 the replacement of an older by a younger form, 

 because the latter is more attractive to the opposite 

 sex, or because it is better concealed, or because the 

 appearance is accompanied by other benefits to the 

 species. The dark variety of the female Silver-washed 

 Fritillary {Argyyinis paphiaj var. valezina), and the 

 white variety of the female Clouded Yellow {Colias 

 edusa, var. hdice), are examples of dimorphism among 

 British butterflies. I exclude that form of dimor- 

 phism, or polymorphism, which is caused by one sex 

 * mimicking ' two or more species which are specially 

 protected (for a good example see pp. 234-38). 



An extremely important form of di- or poly- 

 morphism occurs among the females of the social 

 Hymenoptera. In this case, however, the different 

 forms are specially fitted for certain duties, and the 

 consequent division of labour is beneficial to the 

 society and therefore to the species. 



Seasonal Dimorphism 



Finally, a species which passes through two or 

 more cycles of development in a year, viz. one that 

 is ' double ' or * treble-brooded,' is often characterised 

 by * seasonal dimorphism,' in which the first brood is 



