DIMORPHISM AND POLYMORPHISM 379 



halves. The Termites, in addition to the workers or stunted 

 females, possess 'soldiers' or males, in which the sexual organs 

 are stunted, which possess very strong mandibles, and differ in 

 other important structural details from the ordinary males. In 

 this case, therefore, four determinants must be present, each 

 capable of being substituted for another, and only one of which 

 can be active at a time. 



Apart from local dimorphism, a temporary dimorphism occa- 

 sionally occurs, and is especially well known amongst butterflies 

 as seasonal diinorpJiisui. In this case the individuals of the same 

 generation, hatched nt the same time of year, are alike, but the 

 summer and spring generations differ from one another. 



In the European species Vanessa levana-prorsa, the indi- 

 viduals of the spring generation are characterised by a yellow 

 and black pattern on the upper side of the wings ; while the 

 summer form {prorsa) has black wings, with a broad white 

 transverse band, and delicate yellow lines running parallel to the 

 margins. Were we to superpose these two patterns, it would 

 be seen that the black parts in prorsa do not correspond to 

 the yellow ones in levana, and that the white band in the 

 former does not correspond to a yellow or black part in the 

 latter. This band is, on the contrary, entirely wanting in 

 levana^ and is represented by both black and yellow regions. 



These cases of dimorphism can also, it seems to me, only 

 be accounted for in terms of the idioplasm by the assumption 

 of double-determinants, which, however, are concerned in this 

 case merely with the wing-scales, and essentially with those 

 on the upper side of the wings only ; for the lower surface, 

 though not precisely similar, differs far less in the two forms 

 than does the upper side. We will speak of the halves of these 

 double-determinants as ' winter ' and ' summer ' determinants, 

 and may suppose that the influences of temperature which affect 

 them at the beginning of the pupal stage determine which of 

 the two halves is to predominate over the other. Nearly twenty 

 years ago I showed that it is possible to compel the pupas of the 

 summer-generation to assume the winter form by exposing them 

 to a low temperature, so that the butterfly emerged as a le7>ana 

 instead of a prorsa. The converse experiment was also occa- 

 sionally successful, the pupae of the winter-generation being 

 forced to assume the summer form by the influence of a higher 

 temperature during, or shortly after, pupation. We may per- 



