184 THE EVOLUTION THEORY 



in their full-grown stages. But from what I knew of the ontogeny 

 of Smerintlius species I was able to predict that, among the young 

 .stages of S/jhiux, there must be some with sub-dorsal lines. This 

 was confirmed later, for Poulton found in Sphinx convolvuli that 

 in the first stage there are no oblique stripes, but only the sub-dorsal 

 stripe, while in S2}hinx ligudri both kinds of marking were present 

 at the same time. 



From all these facts, which I have summarized as briefly as 

 possible, we see that the older phyletic characters are gradually 

 crowded by the newer into ever-younger stages in the ontogeny, until 

 ultimately they disappear altogether. We have now to ask to what 

 this phenomenon is due ; is it a simple crowding out of the old and less 

 advantageous by the new and better characters as a result of natural 

 selection, or is there some other factor at work ? It is clear in regard 

 to these forms of marking that they can have been developed at first 

 -only in the almost full-grown larva by natural selection, because they 



Fill. 1 19. Caterpillar of Smerintlms popiili, the Poplar Hawk-moth, at the 

 eud of the first stage, showing both the complete sub-dorsal stripe and the 

 oblique stripes. 



•are of use only there, and that, at the same time, the old marking 

 must have been set aside through the influence of the same factor, in 

 as far as it prejudiced the effect of the new adaptation. This seems 

 to be indicated by the persistence of the sub-dorsal line on those 

 segments which are drawn in when Ghcerocavipa assumes a terrifying 

 attitude, or which do not bear oblique stripes in the leaf-like cater- 

 pillars, e. g. the three anterior segments in the species of Sphinx and 

 Swerinthus. When newly acquired schemes of marking like the eye- 

 .spots of Chairocampa are transmitted from the last stage to the stage 

 before, this can be explained by following the same train of thought, 

 for the caterpillar is already of sufficient size to be able to inspire 

 terror with its eyes ; but in still younger stages the spots would 

 not be likely to have that effect, and yet they occur in quite small 

 animals (20 mm.). More obvious still is the uselessness of the oblique 

 .striping in the young stages of the Sphinx and Smerinthus cater- 

 pillars, for in the earliest stages of life the caterpillars are much too 

 .small to look like a leaf, and the oblique stripes stand much closer 



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