26 ANIMAL COLORATION. 



pillars, caused simply by the slightly altered pigment (chloro- 

 phyll) derived from their food. Possibly the advent of birds, 

 which are after all the chief foes of caterpillars, caused a 

 necessity for some change of colour, to escape their keen sight. 

 Dr. Weismann has pointed out that green, although thoroughly 

 protective as a colour to leaf-feeding caterpillars while they 

 are small, is not so advantageous later : * the large size of 

 the body alone would render them conspicuous ; but this is 

 remedied by longitudinal or oblique striping, which breaks 

 up the large surface into a number of small areas, and thus 

 renders the insect less conspicuous. 



It may therefore be wondered why this particular caterpillar 

 has still retained the primitive coloration ; it is as Weismann 

 has said, " a living fossil." It must always be remembered, 

 however, that animal life is not stationary ; modification must 

 be going on before our eyes ; and the very fact of the rarity 

 of this caterpillar is so far an argument either that it is 

 becoming extinct owing to its inadaptability, or that it is a 

 variety of some form which has really become modified in the 

 required direction. The fact that many Sphinx larvte just 

 before pupation become brown-coloured is considered by Dr. 

 Weismann to be an adaptation to a change in habit : they rest 

 by day, and descend to the ground for concealment ; a green 

 colour would be therefore not nearly so suitable as brown. But 

 we have to consider the fact that such larvae are dimorphic ; 

 some remain green, others turn brown. This, however, is not 

 to be explained, as suggested by Mr. Poulton (p. 135), as a 

 halving of the risks ; some caterpillars resembling leaves and 



It will be noted that this suggestion applies with equal force to 

 tree snakes, frogs, etc., which are usually brought forward as excellent 

 examples of protective coloration (see p. 145) : they are, of course, much 

 larger, and so the need for breaking up the surface is greater. 



