184 THE EVOLUTION THEORY 



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

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

 .stages of Splihix, 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 Sphinx 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 disapjDear 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 



Fig. 119. Caterpillar of Smerinthus 'po2ndi, the Poplar Hawk-motli, at the 

 'eud of the tirst 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 

 isegments which are drawn in when Chcerocavipa 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 Spkiiiix and 

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

 ^spots of Chcerocmn2ya 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 

 teiTor 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 Snierinthus 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 



