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 
