THE COLORATION OF ANIMALS 



73 



stage below, if it were of use there too, and, later on, to still earlier 

 stages in the life-history. 



One might be inclined to ascribe this shunting back of a new 

 character from the later to the earlier stages of development to purely 

 internal forces, which brought it about of necessity, and quite inde- 

 pendently of whether the extension of the character was useful or 

 injurious. We shall come back to this later, and try to find out how 

 far this is the case, but in the meantime we may regard at least so 

 much as established, that this shunting back does not take place 

 everywhere and without limits, but that natural selection calls a halt 

 as soon as its effect would be injurious. 



There could be no continuance of insect-metamorphosis if every 

 character of the final stage had to be shunted back to the one next 

 below, for then, for instance, the characters of the butterfly must, in 



Fig. 8. Caterpillar of the Buckthorn Hawk-moth, Deilephila hippophacs. A, Stage 



III. B, Stage V. r, ring-spots. 



the course of the phyletic evolution, be carried back to the pupa and 

 larva. But even in the larval stage alone it can be seen that this 

 carrying back is kept within well-defined limits. Thus, for instance, 

 in the dimorphic caterpillars of the Sphingidse the brown of the full- 

 grown stage never comes so far down as the earliest stages, for the 

 little caterpillars are all green, like the leaves and stems on which 

 they sit. On the other hand, there are species in which the green 

 persists, as apparently the most advantageous colour. Thus in the 

 buckthorn hawk-moth (Deilephila hippophaes) (Fig. 8), which lives in 

 the warm valleys of the Alps, and especially in Valais, the caterpillars 

 are grey- green in all stages, and are exactly of the shade of the lower 

 surface of the buckthorn leaves ; they possess no oblique lines, for 

 these would not make them more like the leaves, as the full-grown 

 caterpillars are much bigger than an individual leaf of buckthorn, 



