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 AVeismann 

 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 larvse 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 larvEe 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. 



