32 INTRODUCTION 



As a second example of the derivation of patterns, 

 we may mention Viper a aspis, which varies enor- 

 mously as to its mid-dorsal markings, forming, in 

 different individuals or even on different parts of the 

 body, single or paired spots, a zigzag band, or trans- 

 verse bars ; all these are derived from the paired 

 spots of series D. Each pair of spots may fuse and 

 form transversely oval or elliptical spots or bars, or 

 the spots may assume an alternate disposition from 

 which, through confluence, the zigzag or sinuous 

 band results. Thus, spotted and striped patterns 

 may be traced to a common origin, however 

 fundamental the difference between them appears at 

 first sight. If the elements of the four series, D, 

 DL, L, and VL, unite transversely with each other, 

 and also with the spots on the ventral surface, we 

 obtain ringed forms such as the Coral-snakes. 

 That the black nuchal collar of our common Grass- 

 snake is actually formed by the fusion of the spots 

 of three originally distinct series has been proved by 

 tracing the development of the markings in the 

 embryo. 



In various species a pair of light streaks extends 

 along the back, bordering the D area, without inter- 

 fering with the other markings, as we see, among 

 European snakes, in some specimens of Tropidonotus 

 natrix and viperinus, and Vipera bents. 



Although it sometimes happens that a definite 

 system of markings prevails throughout a genus, 



