230 COLOUR IN NATURE chap. 



numerous black spots on a dull gray or yellow-gray 

 ground. This type, in which the very young larvae 

 have dark spots on a pale ground on the upper 

 surface and a pale unspotted lower surface, is ex- 

 ceedingly common among species of Molge. During 

 the course of development the upper surface tends to 

 darken until it presents the appearance of a few light 

 spots on a dark ground, while the primitively uniform 

 lower surface becomes spotted with dark colour. 

 Development, therefore, seems to be accompanied by 

 an increase of dark pigment, and a decrease of yellow 

 or lipochrome pigment. This type, though common, 

 is not universal among the species of Molge ; some, 

 like the common frog, are dark at first and gradually 

 grow lighter. Thus very young larvae of Molge 

 alpestris occur in two varieties. In the one the 

 upper surface is covered with dark -brown spots 

 closely connected together so as to form a network 

 whose meshes are pure brown, while the lower surface 

 is clear and unspotted. The other variety is similar 

 except that the meshes of the network are grayish- 

 yellow instead of brown. As the larvae increase in 

 size, the meshes of the network grow larger and paler 

 in colour until they become grayish-brown or yellow, 

 the network itself at the same time growing paler in 

 colour. Throughout, however, the distinction between 

 pale and dark larvae is preserved, and there is evidence 

 to show that the dark are the future males, and the 

 light the future females. 



The larvae of Molge alpestris like many other 

 larval Amphibians may become sexually mature 

 before metamorphosis. In that case the colours 

 differ slightly from those of the normal adult, the 



