100 THE CAUSES WHICH PROPAGATE 



malti plication of the compound ocellulse and fusion of the per- 

 cipient celhilar structure of each into a sing-le organ connected 

 with a single nerve filament. The eyes of the diurnal butterflies 

 will thus take rank among- the most perfect, both as regards con- 

 vexity and number of facets, which in species amount to as many 

 as 34,650 ; as also in relation to the homogeneity of internal 

 structure. The dragon-flies, with eyes less elevated and circum- 

 ambient, are not far behind, 14,000 facets having been determined 

 in the head of one species. These are vagrant groups, with the 

 sexes often differentiated by bright colour. On the other hand, 

 sluggish species, gregarious from a common food, have generally 

 less development of the visual organs, and rarely complete 

 chromatic differentiation, although brightly coloured, and distin- 

 guishable by pattern. The Plant Bugs are an example. Selec- 

 tion by colour and luminosity is thus in direct relation to the 

 quality of the visual organs ; which, nevertheless, from their 

 enlargement in the raptorial dragon-flies and suctorial flies, con- 

 jointly with an imperfect development in Longicorn Beetles, 

 indicate variation dependent on food as well as display. 



