50 THE VARIATION OF ANIMALS IN NATURE 



increasing in either direction as the humidity increased. By 

 exposing the lightly pigmented form to very humid conditions 

 he was able to show that pigment was slowly acquired through 

 a course of moults, till finally a stage was reached darker than 

 any known in nature. Of other examples of the correlation 



Fig. 2. — Correlation of Yellow Markings with Climatic Conditions in 

 the Wasp Polistes foederata (? above, o* below). The Yellow Markings 

 increase in Warm, Dry Areas. 



(From Zimmermann, 1931.) 



of structural characters and environmental trends one of the 

 best known is that of the number of vertebrae which is associated 

 with a temperature trend in the Atlantic Cod (Schmidt, 1930). 

 The correlation of colour and temperature is also known in 

 insects, e.g. in Polistes (Zimmermann, 1930, 1931). 



(d) Habit -formation. — The habits and instincts of 

 animals are largely responsible for bringing their heredi- 

 tary make-up into play with the environment. No account 



