28 PRIMARY FACTORS OP ORGANIC EVOLUTION. 



understood that, in tracing the derivations from the 

 typical, it is not possible to use one species, as these 

 modifications go on gradually through a number of 

 species, one sometimes beginning where another ends. 



* In the plate, No. i represents vulgaris, which is 

 a fairly typical species, following through generosa 

 (2-3), pamphila (4), hyperborea var. (5), togata (6), gra- 

 tiosa (7), canosa (8), we finally arrive at a perfect white 

 elytron as seen in some varieties of dorsalis. 



" Following in the other direction through tenui- 

 signata (9), marginipennis (10), hentzii (n), sexguttata 

 (12), hcemorrhagica (13), and splendida var. (14), it will 

 be observed that through a gradual thinning or ab- 

 sorption of the markings, or by their fragmentation 

 and obliteration, we arrive at the opposite result of 

 elytra without any white markings whatever, as in 

 many forms of obsoleta, scutellaris, punctulata, and hce- 

 morrhagica. 



' 'Those species which vary from the type in hav- 

 ing the markings broken into spots, as in 12-guttata 

 or hentzii, may lose the spots by a gradual decrease of 

 size, so that they all seem to disappear nearly at the 

 same time ; or the spots may disappear successively, 

 those on the disc being the first to go, while the mar- 

 ginal spots remain. 



"From our series it would be difficult to say which 

 spot is the most persistent, but it is probably the lu- 

 nule, as there are more with entirely dark elytra with 

 slight traces of this spot than with any other, as shown 

 in abdominalis and punctulata. 



"Forms like lemniscata (16) seem very far removed 

 from the type, but many forms of imperfecta (15) show 

 how the markings gradually leave the margin and tend 



