POLYMORPHISM IN SPECIES. 195 



the quality and quantity of food are more important 

 in the variety and extent of the effects dependent 

 upon them. The number of different forms which 

 might thus arise in one species would depend on 

 the number of different regularly recurring condi- 

 tions of development to which the young would 

 be exposed in the long course of generations. 

 Suppose two forms already existing, then we can 

 imagine a new condition to recur periodically in 

 the environment, which, without affecting the con- 

 ditions which produced the first two forms, would 

 by its continued action eventually produce a third 

 form. This new form would produce additional 

 complexity in the social organisation, which might 

 of itself tend to produce a fourth form. It would 

 be impossible to follow out in detail the condi- 

 tions which gave rise, for instance, to the eight 

 forms which exist in a species of white ants 

 (termites). The causes may lie untraceably hidden 

 in the geologically ancient history of the species. 



Suppose a case where, in some such manner as 

 among the butterflies, two forms of ants have devel- 

 oped — one much advanced in structure beyond 

 the other, but both being fertile. Suppose now, 

 while both are produced simultaneously in the 

 species, that through some change in environment 

 the advanced form should become sterile, though 



