374 Studies in the Theory of Descent. 



Finally, among the genera with longitudinal 

 stripes (e.g. Macroglossa) we know certain species 

 which, when at an advanced age, possess oblique 

 stripes (M. Fuciformis], although these slant in a 

 direction opposite to those of most of the other 

 larvae of the Sphingidce. These are, however, 

 always species which differ from their allies in their 

 mode of life, not feeding on grasses or low plants, 

 but on large-leaved shrubs. If we were able to 

 ascertain the ontogeny of these species, we should 

 find that the oblique stripes appeared late in life, 

 as has already been shown in the case of Pterogon 

 CEnotherce. 



If it be asked why the longitudinal lines were 

 first formed, and then the oblique stripes, it may 

 be replied that the physical constitution of these 

 caterpillars would be more easily able to give rise 

 to simple longitudinal lines than to complicated 

 oblique stripes crossing their segments. 5 It may 



6 [The question here also suggests itself as to why the dorsal 

 line should not have been the primary longitudinal stripe, 

 seeing that such a marking is almost naturally produced in 

 many caterpillars by the food in the alimentary canal ; or, in 

 other words, why has not natural selection taken advantage of 

 such an obvious means of producing a stripe in cases where it 

 would have been advantageous? In answer to this I may 

 state, that in large numbers of species the dorsal line has thus 

 become utilized ; but in the case of large caterpillars resting 

 among foliage, it can be easily seen that light lateral (i.e. sub- 

 dorsal) stripes, are more effective in breaking the homogeneity 

 of the body than a dorsal line only slightly darker than the 

 general ground-colour. Lateral lines are in fact visible from 



