xx THE INFLUENCE OF ENVIRONMENT 395 



cold, in making the pupae which should become the 

 summer A. prorxa develop into the winter A. lemma. 

 Nor can we forget the seasonal moulting and the subse- 

 quent change of the plumage in birds, so marked in the 

 case of the ptarmigan, which moults three times in the 

 year. In the puffins even the bill is moulted and appears 

 very different at different seasons. 



Light is very healthful, but it is not easy to explain its 

 precise influence. Our pulses beat faster when we go 

 out into the sunlight. Green plants depend as much on 

 the sunlight as on the soil and the air. It is possible 

 that light has a direct influence on the formation of some 



FIG. 123. SEASONAL CHANGES OF THE BILL IN THE PUFFIN (Fralercnla 

 arclica) ; TO THE LEFT THE SPRING FORM, TO THE RIGHT THE WINTER 

 FORM, BOTH ADULT MALES. 



(After Bureau.) 



animal pigments, as it seems to have in the development 

 of chlorophyll. We know, from Poulton's experiments, 

 that the light reflected from coloured bodies influences the 

 colouring of caterpillars and pupa?, but this influence seems 

 to be subtle and indirect, operating through the nervous 

 system. In the dark caves of Dalmatia, Carniola, and 

 Carinthia there lives a w'ell-known salamander called 

 Proteus, with a wan white skin slightly flushed by the 

 blood. Its skin, as Gadow said, is like a photographic 

 plate, for if the creature be exposed to light it becomes 

 covered with dark spots and eventually quite black. 

 If eggs are laid by individuals kept in the light the newly 

 hatched larvae are dark. This is probably due to the 

 direct influence of the light on the eggs even before 



