PEOTBCTIYE COLORATION. 107 



and-Salt motli and of others bear to a branch of their food- 

 plant are due to natural selection acting in this particular 

 direction. All that we are justified in saying is that these 

 resemblances may have been perfected by that cause. The 

 very fact that the majority of the Geometer caterpillars simulate 

 twigs seems an indication that natural selection has taken 

 advantage of a considerable initial resemblance ; otherwise we 

 might have expected to meet with more mnnerous examples 

 of warning coloration, and of resemblance to other objects, 

 coupled with and heightened by structural as well as colour 

 modifications. 



Occasional Absence of Coloration in Internal-Feeding Caterpillars. 



The fact that internal-feeding caterpillars are white or 

 yellowish has been used as an argument in favour of the 

 effects of reversed natural selection — or Panmixis as it is termed 

 by Weismann. The green colour of the leaf feeders is pro- 

 tective ; and this form of protection is not needed by internal 

 feeders, protected as they are by their position ; hence the 

 green colour tended to disappear and finally did disappear in 

 these caterpillars, because it was useless and no longer pre- 

 served by natural selection ; a green colour came to be not 

 disadvantageous, but simply unnecessary. The caterpillars 

 being no longer selected for their greenness, gradually lost it. 

 Prof. Meldola, however, pointed out that the explanation was 

 in reality the simple one that there was no chlorophyll in 

 their food, and none, therefore, to colour their tissues. Leaf 

 miners, which feed upon the internal tissues of leaves, are often 

 green because these tissues contain chlorophyll. The larva of 

 Nonagria sparganii is green, although it feeds in the interior 

 of Iris stems, and needs no protection by colour. It is pro- 

 bable, though it has, I believe, to be proved, that this green 



